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Glossary · 432 terms · each with its sources

Email terms, defined and sourced

From SPF and DMARC alignment to PTR records and spam traps: what each term means, an example where one helps, and the RFC or provider page it rests on. Wherever a term comes up on this site, its first mention links here.

Every term, A to Z

Type a term, an abbreviation or a word from a definition, or narrow the list to one topic.

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Numbers and symbols

10 terms
  1. -all:The SPF ending that returns fail for any IP address the record does not authorize, the strictest statement a domain can make in SPF.Topic: Authentication
  2. ?all:The SPF ending that returns neutral for unlisted IP addresses, meaning the domain makes no statement either way about whether they may send.Topic: Authentication
  3. +all:The SPF ending that returns pass for every IP address on the internet, authorizing anyone to send mail for the domain.Topic: Authentication
  4. ~all:The SPF ending that returns softfail for unlisted IP addresses: the domain believes they are probably not authorized but stops short of a hard fail.Topic: Authentication
  5. 2xx reply:An SMTP reply starting with 2, meaning the requested step succeeded, such as 220 (ready), 250 (accepted) or 221 (closing).Topic: SMTP
  6. 3xx reply:An SMTP reply starting with 3, meaning the server accepted the command and is waiting for more input, as with 354 after DATA.Topic: SMTP
  7. 4xx reply:An SMTP reply starting with 4, a temporary failure: the server did not accept the step now, and the sender should keep the message and try again later.Topic: SMTP
  8. 5xx reply:An SMTP reply starting with 5, a permanent failure: the server refused the step and the sender should not repeat it unchanged.Topic: SMTP
  9. 7bit:MIME transfer encoding labels meaning the content is not encoded: 7bit is ASCII only in short lines, while 8bit may also contain bytes above 127.Topic: Message format
  10. 8BITMIME:An SMTP extension (RFC 6152) that lets a client send message bodies containing 8-bit bytes, such as raw UTF-8 text, without encoding them first.Topic: SMTP

A

22 terms
  1. A record:The DNS record that maps a host name to an IPv4 address, such as mail.example.com to 192.0.2.25.Topic: DNS
  2. A/B test:Sending two or more versions of an email that differ in one element to parts of a list, and comparing the results to pick the better version.Topic: Sending practice
  3. AAAA record:The DNS record that maps a host name to an IPv6 address, the IPv6 counterpart of the A record.Topic: DNS
  4. Abuse address:The abuse@ mailbox (RFC 2142) where anyone can report spam, phishing or other misuse coming from a domain or network.Topic: Bounces
  5. adkim tag:The DMARC tag choosing relaxed (r, the default) or strict (s) alignment between the DKIM signing domain and the From domain.Topic: Authentication
  6. ADSP:A 2009 DKIM extension, RFC 5617, that let a domain declare it signs all its mail; little used and moved to Historic status in 2013.Topic: Authentication
  7. Allowlist:A list of senders a receiving system trusts and exempts from some filtering. Large mailbox providers generally do not offer senders a way onto one.Topic: Reputation
  8. Anti-spam law:Laws that regulate unsolicited commercial email, such as CAN-SPAM in the US, CASL in Canada and the ePrivacy rules in the EU and UK. They apply by where recipients are.Topic: Compliance
  9. Apple Mail Privacy Protection:An Apple Mail setting that hides the user's IP address from senders and prevents them from seeing whether a message was opened, which breaks pixel-based open tracking.Topic: Providers
  10. ARC:Authenticated Received Chain: signed headers in which each server that handles a message records the authentication results it saw, so later receivers can trust results that forwarding broke.Topic: Authentication
  11. ARC-Authentication-Results:The ARC header in which an intermediary records the SPF, DKIM, DMARC and ARC results it saw on arrival, numbered with the ARC instance.Topic: Authentication
  12. ARC-Message-Signature:The ARC header in which an intermediary signs the message, headers and body, as it passes it on, much like a DKIM signature.Topic: Authentication
  13. ARC-Seal:The ARC header that signs a handler's ARC headers and all earlier ARC sets, carrying the chain validation status cv= of none, pass or fail.Topic: Authentication
  14. ARF:The Abuse Reporting Format of RFC 5965: a machine-readable email format that mailbox providers use to send spam complaints and other feedback to senders.Topic: Reputation
  15. aspf tag:The DMARC tag choosing relaxed (r, the default) or strict (s) alignment between the SPF-authenticated MAIL FROM domain and the From domain.Topic: Authentication
  16. Asynchronous bounce:A bounce that arrives later as a separate email, because a server first accepted the message and only afterwards found it could not deliver it.Topic: Bounces
  17. Attachment:A file sent inside an email as its own MIME part, usually base64-encoded and marked Content-Disposition: attachment with a file name.Topic: Message format
  18. Authentication-Results:The header a receiving server adds to record the results of its SPF, DKIM, DMARC and other checks, for filters and for anyone reading the headers.Topic: Authentication
  19. Author Domain:DMARC's name for the domain in the message's From header, the address the reader sees and the domain whose policy DMARC applies.Topic: Authentication
  20. Authoritative nameserver:A DNS server that holds a zone's records itself and answers for them with authority, as listed in the zone's NS records.Topic: DNS
  21. Auto-reply:A message sent automatically in response to an incoming email, such as an out-of-office notice, a ticket acknowledgement or a mailing list command reply.Topic: Message format
  22. Auto-Submitted:The header field (RFC 3834) that marks a message as generated automatically, with the value auto-generated or auto-replied, so other automatic systems do not respond to it.Topic: Message format

B

17 terms
  1. B2B email prospecting:Sending commercial email to people at their work addresses. Rules differ by country: France and the UK allow it without prior consent under conditions, CAN-SPAM requires none.Topic: Compliance
  2. Backscatter:Bounces, auto-replies and other notices sent to an innocent address that spammers forged as the sender, flooding it with reports about mail it never sent.Topic: Bounces
  3. Barracuda Reputation Block List:Barracuda Networks' IP reputation list, maintained by Barracuda Central from spam traps and other systems and used by its email gateways to block poorly rated senders.Topic: Reputation
  4. Base64:A MIME transfer encoding that writes any bytes using 64 safe ASCII characters, four characters for every three bytes; used for attachments and images.Topic: Message format
  5. Bayesian filtering:A statistical spam-filtering method that learns from mail already classified as spam or not, and scores new messages by the tokens they contain.Topic: Reputation
  6. Bcc:Blind carbon copy: recipients who receive the message while their addresses stay hidden from the other recipients.Topic: Message format
  7. BIMI:Brand Indicators for Message Identification: a DNS record pointing to a logo that supporting inboxes display next to mail that passes DMARC at enforcement.Topic: Authentication
  8. BIMI record:The TXT record at default._bimi followed by the domain, starting v=BIMI1, giving the logo URL (l=) and the mark certificate URL (a=).Topic: Authentication
  9. Block bounce:A rejection caused by the sender rather than the address: the receiver refused the message because of IP or domain reputation, a blocklist listing, authentication or content.Topic: Bounces
  10. Blocklist:A list of IP addresses, domains or hashes judged to send spam or abuse, which receiving systems query to block or score mail. Blacklist is the older name.Topic: Reputation
  11. Bounce:An email that could not be delivered and came back to the sender, either refused during the SMTP session or reported later in a non-delivery report.Topic: Bounces
  12. Bounce handling:Receiving, parsing and acting on bounces automatically: classifying each failure and suppressing, retrying or flagging the address accordingly.Topic: Bounces
  13. Bounce message:The email a sender receives when a message could not be delivered, typically from MAILER-DAEMON or postmaster, naming the recipient that failed and why.Topic: Bounces
  14. Bounce rate:The share of sent messages that bounced, counted by each platform in its own way; a high rate signals a poorly kept list to providers and sending platforms.Topic: Bounces
  15. Bulk complaint level:A 0 to 9 rating Microsoft 365 gives mail from bulk senders, higher when the sender draws more complaints. Mail at or above the threshold goes to junk by default.Topic: Providers
  16. Bulk sender:At Gmail, any sender that sends close to 5,000 messages or more to personal Gmail accounts within 24 hours, a status Google says is permanent once reached.Topic: Sending practice
  17. Business email compromise:Fraud in which criminals impersonate or take over a business email account to trick staff or partners into transferring money or sensitive data.Topic: Security

C

21 terms
  1. CAA record:The DNS record naming which certificate authorities may issue certificates for a domain; authorities must check it before issuing.Topic: DNS
  2. CAN-SPAM:The US federal law on commercial email, enforced by the FTC. It requires no prior consent but sets rules on honesty, identification, a postal address and opt-out.Topic: Compliance
  3. CASL:Canada's anti-spam law: commercial electronic messages need express or implied consent, sender identification and an unsubscribe that works within 10 business days.Topic: Compliance
  4. Catch-all address:A mailbox that receives mail sent to any address at a domain that matches no real user, so the domain's server accepts every recipient instead of rejecting unknown ones.Topic: Bounces
  5. Cc:Carbon copy: the header field listing secondary recipients who receive a copy of the message, visible to everyone who gets it.Topic: Message format
  6. Charset:The Content-Type parameter naming how a text part's bytes map to characters, such as utf-8 or iso-8859-1; a wrong value garbles accented and non-Latin text.Topic: Message format
  7. CHUNKING:An SMTP extension (RFC 3030) that sends the message in counted chunks with BDAT instead of DATA, so no end-of-data dot or dot-stuffing is needed.Topic: SMTP
  8. Click rate:The share of delivered emails in which at least one tracked link was clicked. Security tools that follow links can register clicks that no person made.Topic: Reputation
  9. Click tracking:Rewriting each link in an email to pass through the sender's tracking server, which records the click and then redirects the reader to the real destination.Topic: Message format
  10. CNAME record:The DNS record that makes one name an alias of another, so lookups for the alias continue at the target name.Topic: DNS
  11. CNIL:France's data protection authority, which supervises the GDPR in France and the French rules on email prospecting, and publishes guidance on both.Topic: Compliance
  12. Cold email:An unsolicited email from an identifiable sender to someone with no prior relationship or consent, seeking a sale, meeting or other business benefit.Topic: Sending practice
  13. Commercial email:Under anti-spam laws, a message whose purpose is to promote a product, service or business opportunity. Such messages carry the consent, identification and opt-out duties.Topic: Compliance
  14. Common Mark Certificate:A BIMI mark certificate for a logo that is not a registered trademark, validated on prior use of the mark or as a modified registered mark.Topic: Authentication
  15. compauth:Microsoft 365's composite authentication result, combining SPF, DKIM, DMARC and other signals for the From domain into pass, fail, softpass or none.Topic: Authentication
  16. Complaint rate:The share of your mail that recipients report as spam. Gmail and Yahoo both publish a 0.3% limit, and Google recommends staying below 0.1%.Topic: Reputation
  17. Connection limit:The number of simultaneous SMTP connections a sender opens to one destination, or a receiver allows from one sender, beyond which connections are refused or deferred.Topic: SMTP
  18. Content filtering:Spam filtering that judges a message by what it contains (text, links, attachments, formatting) rather than only by who sent it and from where.Topic: Reputation
  19. Content-Disposition:The MIME header (RFC 2183) that suggests whether a part should be shown inline in the message or offered as an attachment, and under what file name.Topic: Message format
  20. Content-Transfer-Encoding:The MIME header that states how a part's bytes were encoded for transport: 7bit, 8bit, binary, quoted-printable or base64.Topic: Message format
  21. Content-Type:The MIME header that states what a message or part contains, such as text/plain; charset=utf-8, text/html or multipart/mixed, with parameters like the charset or the boundary.Topic: Message format

D

60 terms
  1. DANE:DNS-Based Authentication of Named Entities: publishing a mail server's certificate fingerprint in DNSSEC-signed TLSA records so senders can require and verify TLS.Topic: Security
  2. DATA command:The SMTP command after which the client sends the message itself, header and body, ending with a line that holds a single dot.Topic: SMTP
  3. Date header:The required header field recording when the author's program finished the message and released it for sending, with a numeric time zone offset.Topic: Message format
  4. Dedicated IP:A sending IP address used by one sender only, so its reputation reflects that sender's mail alone. It needs steady volume and a warm-up before use.Topic: Sending practice
  5. Deferral:A delivery attempt that met a temporary 4xx failure, leaving the message in the sender's queue to be retried later instead of bounced.Topic: Bounces
  6. Delay notification:A warning sent to the sender while a message is still being retried, saying delivery is delayed but the server will keep trying; it is not a bounce.Topic: Bounces
  7. Delisting:Getting an IP address or domain removed from a blocklist, either by waiting for the listing to expire or by requesting removal once the cause is fixed.Topic: Reputation
  8. Deliverability:The ability of your email to reach the recipient's inbox rather than being rejected, deferred or filed in spam, as each mailbox provider decides.Topic: Reputation
  9. Delivered-To header:A header recording the address a message was actually delivered to, added by the receiving system at each delivery step, which helps where forwarding or aliases are involved.Topic: Message format
  10. Delivery:In sending reports, usually a receiving server accepting a message with a 250 reply after the message data; it says nothing about whether the message reached the inbox or the spam folder.Topic: Bounces
  11. Delivery rate:The share of sent messages that receiving servers accepted, counting a message as delivered even when the provider then files it in the spam folder.Topic: Reputation
  12. Delivery timeout:How long a sending server keeps retrying a deferred message before it gives up and returns a bounce; RFC 5321 suggests at least four to five days.Topic: SMTP
  13. Display name:The human-readable name shown with an email address, as in Ana Silva <ana@example.com>; most mail programs show it in place of the address.Topic: Message format
  14. Display-name spoofing:Putting a trusted name in the display-name part of the From header over an unrelated address, such as "Example Bank" <random@freemail.example>.Topic: Security
  15. Disposable email address:A temporary address used briefly and then abandoned, often from a public inbox service that needs no signup, typically to get past a form without sharing a real address.Topic: Sending practice
  16. DKIM:DomainKeys Identified Mail: a cryptographic signature added to a message by a domain that takes responsibility for it, verified with a public key in that domain's DNS.Topic: Authentication
  17. DKIM alignment:The DMARC condition met when a valid DKIM signature's d= domain matches the From domain, by organizational domain or exactly.Topic: Authentication
  18. DKIM b= tag:The required tag in a DKIM signature holding the base64 signature value, computed over the signed header fields and the DKIM-Signature header itself.Topic: Authentication
  19. DKIM bh= tag:The required tag in a DKIM signature holding the base64 hash of the canonicalized message body, checked before the signature itself.Topic: Authentication
  20. DKIM canonicalization:How a DKIM signer normalizes headers and body before hashing, simple or relaxed for each, set in the c= tag so small changes in transit do not break the signature.Topic: Authentication
  21. DKIM d= tag:The tag in a DKIM signature naming the signing domain, which takes responsibility for the message and whose DNS holds the public key.Topic: Authentication
  22. DKIM h= tag:The required tag in a DKIM signature listing, in order, the header fields covered by the signature; the From field must always be on it.Topic: Authentication
  23. DKIM i= tag:The optional tag in a DKIM signature naming the agent or user the signer acts for; its domain must equal or be a subdomain of d=.Topic: Authentication
  24. DKIM key length:The size of the RSA key behind a DKIM signature; RFC 8301 requires at least 1024 bits and recommends at least 2048.Topic: Authentication
  25. DKIM key rotation:Replacing a DKIM signing key with a new one under a new selector, keeping both published until mail signed with the old key has been verified.Topic: Authentication
  26. DKIM l= tag:The optional DKIM tag stating how many bytes of the body the signature covers; RFC 6376 warns it lets content be appended unseen.Topic: Authentication
  27. DKIM oversigning:Listing a header field in DKIM's h= tag more times than it appears, so that adding another copy of that field after signing breaks the signature.Topic: Authentication
  28. DKIM record:The TXT record at selector._domainkey.domain that publishes a DKIM public key, with tags such as v=DKIM1, k for the key type and p for the key.Topic: Authentication
  29. DKIM replay:An abuse in which someone obtains one message validly signed by a reputable domain and resends it to many recipients, borrowing that domain's reputation.Topic: Authentication
  30. DKIM s= tag:The required tag in a DKIM signature that names the selector, telling the verifier where under the d= domain to fetch the public key.Topic: Authentication
  31. DKIM selector:The name in a DKIM signature's s= tag that tells receivers which of a domain's public keys to fetch, at selector._domainkey.domain.Topic: Authentication
  32. DKIM signature:The DKIM-Signature header a signer adds to a message, holding the signing domain, selector, algorithm, signed header list, body hash and signature.Topic: Authentication
  33. DMARC:A DNS-published policy that ties SPF and DKIM to the visible From domain, tells receivers how to treat mail that fails, and asks them for reports.Topic: Authentication
  34. DMARC aggregate report:The daily XML summary a receiver sends to a domain's rua address, counting messages by sending IP address with their SPF, DKIM and DMARC results.Topic: Authentication
  35. DMARC alignment:The DMARC requirement that the domain SPF or DKIM authenticated matches the From domain, exactly (strict) or by organizational domain (relaxed).Topic: Authentication
  36. DMARC enforcement:The state in which a domain's DMARC policy, for the organizational domain and all subdomains, is quarantine or reject rather than none.Topic: Authentication
  37. DMARC failure report:A per-message report a receiver may send to a domain's ruf address when a message fails DMARC, often with its headers or full content.Topic: Authentication
  38. DMARC policy:The p= value in a DMARC record, none, quarantine or reject, stating how the domain owner wants receivers to treat mail that fails DMARC.Topic: Authentication
  39. DMARC record:The TXT record at _dmarc followed by the domain, beginning v=DMARC1, that carries a domain's DMARC policy and reporting addresses.Topic: Authentication
  40. DMARC t= tag:The DMARC tag introduced by RFC 9989 to mark a policy as being tested: t=y asks receivers to apply one level less than the stated policy.Topic: Authentication
  41. DMARCbis:The IETF revision of DMARC published in May 2026 as RFC 9989, 9990 and 9991, which made DMARC a standards-track protocol and replaced RFC 7489.Topic: Authentication
  42. DNS:The Domain Name System: the distributed database that turns domain names into IP addresses and holds the records mail depends on, such as MX, TXT and PTR.Topic: DNS
  43. DNS host:The provider whose authoritative nameservers serve a domain's zone, where its records (MX, SPF, DKIM, DMARC) are actually created and edited.Topic: DNS
  44. DNS over HTTPS:Sending DNS queries and answers inside HTTPS (RFC 8484), so they are encrypted and can be made from places like browsers and web workers.Topic: DNS
  45. DNS over TLS:Sending DNS queries over a TLS-encrypted TCP connection to port 853 (RFC 7858), protecting lookups between a client and its resolver.Topic: DNS
  46. DNS propagation:The common name for the delay before a DNS change is seen everywhere, caused by resolvers caching old answers until their TTLs expire.Topic: DNS
  47. DNS record:One entry in DNS: an owner name, a type such as A, MX or TXT, a class, a TTL and the data, published in a domain's zone.Topic: DNS
  48. DNS resolver:The server that looks DNS names up on behalf of clients, querying authoritative servers from the root down and caching the answers.Topic: DNS
  49. DNS tree walk:RFC 9989's method for finding a DMARC policy and organizational domain by querying _dmarc records up the domain name, at most eight queries.Topic: Authentication
  50. DNS zone:The part of the DNS namespace one set of authoritative servers is responsible for, such as example.com and the names below it that are not delegated elsewhere.Topic: DNS
  51. DNSBL:A blocklist published in the DNS: a receiving server looks up the connecting IP (or a domain) in the list zone, and an answer means it is listed.Topic: Reputation
  52. DNSSEC:DNS Security Extensions: digital signatures on DNS records that let resolvers verify an answer came from the zone owner and was not altered.Topic: DNS
  53. Domain registrar:The company through which a domain name is registered and renewed, which passes your nameserver and DNSSEC settings to the registry.Topic: DNS
  54. Domain reputation:The reputation a mailbox provider attaches to a domain in your mail, chiefly the authenticated sending domain, which stays with you when you change IP addresses.Topic: Reputation
  55. Domain warm-up:Introducing a new sending domain or subdomain to mailbox providers with gradually rising volume, so it earns a reputation instead of being judged on a sudden spike.Topic: Sending practice
  56. DomainKeys:The Yahoo-designed predecessor of DKIM, documented in 2007 as Historic RFC 4870, which signed messages with a DomainKey-Signature header.Topic: Authentication
  57. Dot-stuffing:The SMTP rule that doubles a dot at the start of any line of message text, so that a line holding a single dot can safely mark the end of the message.Topic: SMTP
  58. Double opt-in:A signup that is only complete once the subscriber clicks a link or replies to a confirmation email, proving the address is theirs and the request was real.Topic: Sending practice
  59. DSN:A Delivery Status Notification: the standard machine-readable report (RFC 3464) on what happened to a message, used for bounces, delays and success notices.Topic: Bounces
  60. DSN extension:An SMTP extension (RFC 3461) with which a sender asks for delivery status notifications per recipient: on failure, on delay, on success, or never.Topic: SMTP

E

29 terms
  1. Ed25519 for DKIM:The elliptic-curve signing algorithm added to DKIM by RFC 8463 in 2018, identified as a=ed25519-sha256 with a key record of k=ed25519.Topic: Authentication
  2. EHLO:The command an SMTP client sends first, giving its own hostname and asking the server to list the SMTP extensions it supports.Topic: SMTP
  3. Email address internationalization:The standards (RFC 6530 to 6533) that allow email addresses and headers in any script, such as Chinese, Arabic or accented Latin, encoded in UTF-8.Topic: Message format
  4. Email alias:An extra address that delivers into an existing mailbox or expands to other addresses, without being a mailbox of its own.Topic: Bounces
  5. Email appending:Matching existing customer records against a third-party database to add email addresses the customers never gave you. M3AAWG calls it abusive.Topic: Sending practice
  6. Email client:The program or app a person uses to read and write email, such as Outlook, Apple Mail, Thunderbird or the Gmail app; in standards terms, a Mail User Agent.Topic: Message format
  7. Email engagement:How recipients interact with your mail: opening, clicking, replying, rescuing it from spam, deleting it unread or reporting it. Providers use these signals in filtering.Topic: Reputation
  8. Email forwarding:Automatically sending mail received at one address on to another, which changes the path a message takes and often breaks SPF, while DKIM usually survives.Topic: Bounces
  9. Email header:The block of header fields at the top of an email message, holding sender, recipients, date, subject, routing trace and authentication results, separated from the body by a blank line.Topic: Message format
  10. Email header injection:An attack on a web form or app that builds email from user input: injected line breaks add headers such as Bcc, turning the form into a spam relay.Topic: Security
  11. Email marketing:Email sent to promote products or build the relationship with recipients, as opposed to transactional mail. It usually needs prior consent or an opt-out, depending on the law.Topic: Sending practice
  12. Email sequence:A series of timed emails sent to a contact one after another, typically stopping when the contact replies, used in sales outreach and cold email.Topic: Sending practice
  13. Email sequencer:Software that sends email sequences from connected mailboxes on a schedule, inserts personal details, stops on replies and reports results per sequence.Topic: Sending practice
  14. Email service provider:A company that sends email at volume on behalf of its customers, providing the sending infrastructure, list management and reporting. Not the same as a mailbox provider.Topic: Sending practice
  15. Email spoofing:Sending mail that falsely claims to come from someone else, usually by forging the From address with a domain the sender does not control.Topic: Security
  16. Email threading:Grouping an email and its replies into one conversation in the mail program, using the In-Reply-To and References fields and the subject line.Topic: Message format
  17. Email verification:Checking whether an email address is valid and can receive mail before sending to it: syntax, domain and MX records, and sometimes a probe of the receiving server.Topic: Sending practice
  18. Encoded-word:The RFC 2047 syntax for putting non-ASCII text into email headers, written =?charset?encoding?text?=, used for subjects and display names with accents or other scripts.Topic: Message format
  19. Enhanced status code:A three-part code such as 5.1.1 or 4.7.0 (RFC 3463) that accompanies an SMTP reply code and gives the reason more precisely: class, subject and detail.Topic: SMTP
  20. Envelope recipient:An address given in an SMTP RCPT TO command: where the message is actually delivered, whatever its To and Cc headers say.Topic: SMTP
  21. Envelope sender:The address given in the SMTP MAIL FROM command, where bounces and delivery reports are sent; also called the return path or bounce address.Topic: SMTP
  22. ePrivacy Directive:EU Directive 2002/58/EC on privacy in electronic communications. Its Article 13 requires prior consent for marketing email to individuals, with a soft opt-in for existing customers.Topic: Compliance
  23. ESMTP:Extended SMTP: SMTP with service extensions, where a server answers EHLO with a list of extra capabilities, such as SIZE, PIPELINING or STARTTLS, that the client may then use.Topic: SMTP
  24. Exchange Online Protection:The filtering included with every Microsoft 365 cloud mailbox (anti-malware, anti-spam and anti-phishing), which Microsoft now documents as built-in security features for all cloud mailboxes.Topic: Providers
  25. exists mechanism:The SPF mechanism that builds a domain name, usually from macros, and matches if that name has any A record, whatever the address.Topic: Authentication
  26. exp modifier:The SPF modifier exp=domain, which points to a TXT record holding the explanation text a receiver may return when a message fails SPF.Topic: Authentication
  27. EXPN:An SMTP command that asks a server to expand a mailing list or alias into its member addresses; public servers almost always refuse it.Topic: SMTP
  28. Express consent:Permission a person gives actively and knowingly before receiving marketing mail, such as by ticking an unchecked box. EU law requires it for marketing email to individuals.Topic: Compliance
  29. External destination verification:The check that a DMARC report address outside the domain's organization agreed to receive its reports, via a TXT record under _report._dmarc.Topic: Authentication

F

7 terms
  1. FCrDNS:Forward-confirmed reverse DNS: the IP address's PTR name must itself resolve, through A or AAAA records, back to the same IP address.Topic: DNS
  2. Feedback loop:A service in which a mailbox provider reports back to a sender each time one of its recipients marks a message as spam, so the complainant can be removed.Topic: Reputation
  3. Feedback-ID:A header Gmail reads to report spam complaint rates per campaign or customer in Postmaster Tools: up to three identifiers and a sender ID, separated by colons.Topic: Message format
  4. fo tag:The DMARC tag choosing when failure reports should be generated: 0, 1, d, s or a combination; it is ignored if ruf is absent.Topic: Authentication
  5. Follow-up email:A later message in a sequence sent to someone who has not replied to an earlier one, usually as a reply in the same thread.Topic: Sending practice
  6. FQDN:A fully qualified domain name: a complete name down to the root, such as mx1.example.com, as opposed to a short relative name like mx1.Topic: DNS
  7. From header:The header field naming the message's author: the address and display name a reader sees as the sender, and the domain DMARC protects.Topic: Message format

G

11 terms
  1. GDPR:The EU's General Data Protection Regulation (2016/679), applied since 25 May 2018. An email address that identifies a person is personal data, so mailing it needs a lawful basis.Topic: Compliance
  2. Gmail:Google's email service, for personal accounts and for businesses through Google Workspace. Its sender guidelines and Postmaster Tools set much of the industry's practice.Topic: Providers
  3. Gmail clipping:Gmail cutting a long email short and showing [Message clipped] with a link to the full message, which senders observe at about 102 KB of HTML.Topic: Message format
  4. Gmail Feedback Loop:Gmail's campaign-level feedback loop: senders tag mail with a Feedback-ID header, and Postmaster Tools reports identifiers that draw unusual spam rates from Gmail users.Topic: Providers
  5. Gmail sender guidelines:Google's requirements for sending to personal Gmail accounts, in force since February 2024: one set for all senders and stricter rules for those sending 5,000 or more a day.Topic: Providers
  6. Gmail spam rate thresholds:Google's published limits for user-reported spam: keep the rate shown in Postmaster Tools below 0.10% and never let it reach 0.30%.Topic: Providers
  7. Gmail tabs:The categories Gmail's default inbox sorts mail into: Primary, Social, Promotions, Updates and Forums. A message in a tab has reached the inbox.Topic: Providers
  8. Google Postmaster Tools:Google's free dashboards for senders to Gmail: compliance with the sender guidelines, user-reported spam rate, IP and domain reputation, authentication, encryption and delivery errors.Topic: Providers
  9. Google Workspace:Google's paid productivity suite for organisations, formerly G Suite, which hosts business email on Gmail under the organisation's own domain.Topic: Providers
  10. Graymail:Microsoft's name for bulk mail that recipients did sign up for but often do not want, rated with the bulk complaint level and filed in junk above a threshold.Topic: Reputation
  11. Greylisting:An anti-spam technique where a receiving server temporarily refuses mail from a sender it has not seen before and accepts it when the sending server retries later.Topic: SMTP

H

9 terms
  1. Hard bounce:A permanent delivery failure for a reason that will not change on retry, typically because the address or its domain does not exist.Topic: Bounces
  2. Header field:One entry in an email header: a field name, a colon and a value, such as Subject: Your invoice, possibly folded across several lines.Topic: Message format
  3. Header folding:Splitting a long header field over several lines by inserting a line break before a space or tab; programs undo it by joining the lines back together.Topic: Message format
  4. HELO:The original SMTP greeting command from RFC 821: it identifies the client to the server but cannot negotiate any SMTP extensions.Topic: SMTP
  5. HELO hostname:The hostname a sending server gives in its EHLO or HELO command, which receivers record in the Received header and often check against DNS and the connecting IP.Topic: SMTP
  6. HELO identity:The host name a sending server gives in its SMTP HELO or EHLO greeting, which SPF can check against that name's own SPF record.Topic: Authentication
  7. Homograph attack:A lookalike-domain attack that uses characters from other scripts which look identical to Latin letters, such as a Cyrillic a in place of a Latin a.Topic: Security
  8. Honeypot:A decoy set out to attract abusers, in email usually addresses published where only address harvesters will find them, so any mail they receive identifies a spammer.Topic: Reputation
  9. HTML email:An email whose body is written in HTML (text/html), allowing layout, styling, images and links, rendered by each mail program with its own limited HTML and CSS support.Topic: Message format

I

20 terms
  1. iCloud Mail:Apple's email service for icloud.com, me.com and mac.com addresses. Apple publishes bulk sending requirements but offers no feedback loop and no allowlist.Topic: Providers
  2. ICO:The UK's Information Commissioner's Office, the regulator for UK GDPR and for PECR, which governs marketing email in the UK.Topic: Compliance
  3. Image blocking:Mail programs not loading an email's remote images until the reader allows it, either by default, by the reader's setting, or because the sender looks suspicious.Topic: Message format
  4. Implicit MX:The SMTP rule that when a domain has no MX records, senders deliver to the domain's own A or AAAA address as if it had an MX of preference 0.Topic: DNS
  5. Implied consent:Permission inferred from a relationship or action, such as a purchase, rather than given expressly. CASL defines when it applies; M3AAWG advises against relying on it.Topic: Compliance
  6. In-Reply-To header:The header field in a reply that holds the Message-ID of the message being answered, so mail programs can link the two.Topic: Message format
  7. Inbox placement:Where an accepted message ends up for the recipient: the inbox (or one of its tabs) or the spam folder, a choice the mailbox provider makes and does not report back.Topic: Reputation
  8. Inbox placement rate:The percentage of test messages that reached the inbox rather than spam or nowhere, usually measured by sending to a seed list of mailboxes at several providers.Topic: Reputation
  9. Inbox placement test:A test that sends a message to a seed list and reports, per provider, whether each copy reached the inbox, a tab, the spam folder or nothing.Topic: Reputation
  10. Include mechanism:The SPF mechanism include:domain, which runs the SPF check against another domain's record and matches if that check returns pass.Topic: Authentication
  11. Inline image:An image displayed within the body of an HTML email, either embedded in the message as a MIME part referenced by cid: or loaded from a web address.Topic: Message format
  12. Internationalized domain name:A domain name containing non-ASCII characters, such as accented or non-Latin letters, stored in DNS in an ASCII form beginning with xn--.Topic: DNS
  13. Internet Message Format:The standard for the text of an email message, RFC 5322: header fields such as From, To, Date and Subject, then a blank line, then the body.Topic: Message format
  14. Invalid email address:An address that cannot receive mail, because it is malformed, its domain does not exist or accept mail, or the mailbox does not exist. Sending to it causes a hard bounce.Topic: Sending practice
  15. IP pool:A group of sending IP addresses managed together, typically one pool per type of mail, so the reputation of one stream does not affect another.Topic: Sending practice
  16. IP reputation:The reputation a mailbox provider or blocklist attaches to a sending IP address, based on the mail and behaviour seen from that address.Topic: Reputation
  17. IP warm-up:Raising the volume sent from a new IP address gradually, starting with the most engaged recipients, so providers can build a reputation for it before full volume.Topic: Sending practice
  18. ip4 mechanism:The SPF mechanism that matches an IPv4 address or network written directly in the record, such as ip4:192.0.2.0/24, without any DNS lookup.Topic: Authentication
  19. ip6 mechanism:The SPF mechanism that matches an IPv6 address or network written directly in the record, such as ip6:2001:db8::/32, without any DNS lookup.Topic: Authentication
  20. IPv6 sending:Delivering mail from an IPv6 address, which large receivers accept only from senders with valid reverse DNS and passing authentication.Topic: SMTP

J

1 term
  1. JMRP:Microsoft's complaint feedback loop for Outlook.com: senders who enroll receive a report when Outlook.com users mark their messages as junk.Topic: Providers

L

13 terms
  1. Legitimate interest:A GDPR lawful basis allowing processing that a business needs for its own interests, if those are not overridden by the person's rights. Direct marketing can qualify.Topic: Compliance
  2. Line length limit:The rule that each line of an email message may hold at most 998 characters plus the CRLF (RFC 5322), and should hold no more than 78.Topic: Message format
  3. Link shortener:A public service that turns long URLs into short redirect links. In email its shared domain hides the destination and can carry other users' bad reputation.Topic: Sending practice
  4. Link wrapping:Replacing a link's real address with one that passes through another server first, done by senders for click tracking and by receivers' security systems to check links when clicked.Topic: Message format
  5. List bombing:An attack in which a script submits one victim's address to many signup forms, flooding that inbox with confirmation and welcome emails from legitimate senders.Topic: Sending practice
  6. List decay:The steady loss of accuracy in a mailing list as people change jobs and abandon addresses, turning valid addresses into bounces and, later, recycled spam traps.Topic: Sending practice
  7. List headers:The List- header fields of RFC 2369 (List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive) that describe how to use a mailing list.Topic: Message format
  8. List hygiene:Keeping a mailing list to people who asked for mail and still read it: removing bounces, complainants, unsubscribes and long-inactive addresses.Topic: Sending practice
  9. List-Id:A header field (RFC 2919) that gives a mailing list a stable identifier shaped like a domain name, so mail programs and filters can recognise mail from that list.Topic: Message format
  10. List-Unsubscribe:A header field (RFC 2369) giving mail programs a way to unsubscribe the reader, as a mailto: address, an https: link or both; it powers the unsubscribe button in Gmail and others.Topic: Message format
  11. List-Unsubscribe-Post:The header field (RFC 8058) that turns a List-Unsubscribe https link into a one-click unsubscribe; its only allowed value is List-Unsubscribe=One-Click.Topic: Message format
  12. Local part:The part of an email address before the @, which only the receiving domain may interpret; RFC 5321 limits it to 64 octets.Topic: Message format
  13. Lookalike domain:A domain registered to resemble a real one, such as examp1e.com or example-billing.com, so that mail from it is mistaken for mail from the genuine organization.Topic: Security

M

31 terms
  1. MAIL FROM command:The SMTP command that opens a mail transaction and gives the envelope sender, the address that bounces and delivery reports are sent back to.Topic: SMTP
  2. Mail loop:A message passed around in a circle between servers or forwarding rules, which servers detect by counting Received headers and stop with a bounce.Topic: SMTP
  3. Mail queue:The store where a mail server keeps messages it has accepted but not yet delivered, retrying them until they go through or expire.Topic: SMTP
  4. Mail relay:A server that receives a message over SMTP and passes it on to another server instead of delivering it into a local mailbox.Topic: SMTP
  5. Mail server:A computer running software that sends, receives or stores email: a submission server, an MTA relaying mail, an MX host receiving it, or a mailbox server.Topic: SMTP
  6. Mail stream:A category of mail sent with its own identity (IP, DKIM domain or subdomain, From address), such as marketing kept apart from transactional mail, so each builds its own reputation.Topic: Sending practice
  7. Mailbox full:A delivery failure because the recipient's mailbox has reached its storage limit, reported with enhanced status code 4.2.2 or 5.2.2.Topic: Bounces
  8. Mailbox provider:A company that hosts email mailboxes for people or organisations, such as Gmail, Outlook.com, Yahoo Mail or iCloud Mail, and decides what reaches them.Topic: Sending practice
  9. Mailing list:An address that redistributes each message it receives to all its subscribers, changing the envelope sender so that bounces return to the list rather than to the author.Topic: Bounces
  10. Mark certificate:An X.509 certificate that binds a logo to a domain for BIMI after a certification authority checks the right to use the mark; VMCs and CMCs are the two kinds.Topic: Authentication
  11. MDA:A Mail Delivery Agent: the component that takes a message from the last MTA and places it in the recipient's mailbox, carrying out any delivery rules the recipient has set.Topic: SMTP
  12. Merge tag:A placeholder in an email template, such as *|FNAME|* in Mailchimp, that the sending platform replaces with each recipient's stored data at send time.Topic: Sending practice
  13. Message body:Everything in an email after the header and the blank line that ends it: the text, HTML and attachments, structured by MIME when there is more than plain text.Topic: Message format
  14. Message submission:The step where a mail program or sending tool hands a new message to its own provider's server, defined by RFC 6409 as a service separate from relay between servers.Topic: SMTP
  15. Message-ID:A header field holding a globally unique identifier for one message, such as <20261020.4f2a9c@mail.example.com>, used for threading, duplicate detection and tracing.Topic: Message format
  16. Microsoft 365:Microsoft's business suite, whose email runs on Exchange Online and is filtered by Exchange Online Protection, separately from the consumer Outlook.com service.Topic: Providers
  17. Microsoft Defender for Office 365:Microsoft's paid email and collaboration security for Microsoft 365, in Plan 1 and Plan 2, which adds protections such as Safe Links and Safe Attachments to the built-in filtering.Topic: Providers
  18. MIME:Multipurpose Internet Mail Extensions: the standards (RFC 2045 to 2049) that let email carry other character sets, HTML, images, attachments and messages made of several parts.Topic: Message format
  19. MIME boundary:The string, declared in a multipart Content-Type, that marks where each part of a MIME message starts and where the last one ends.Topic: Message format
  20. MSA:A Mail Submission Agent: the server that accepts new mail from a user's program, checks and completes it, and hands it to the mail system for delivery.Topic: SMTP
  21. MTA:A Mail Transfer Agent: server software that receives messages over SMTP and relays them toward their destination, such as Postfix, Exim or Microsoft Exchange.Topic: SMTP
  22. MTA-STS:SMTP MTA Strict Transport Security: a policy, signalled in DNS and served over HTTPS, telling sending servers to deliver to a domain only over authenticated TLS.Topic: Security
  23. MTA-STS mode:The mode line of an MTA-STS policy: enforce refuses delivery that fails validation, testing only reports failures, and none withdraws the policy.Topic: Security
  24. MTA-STS policy:The text file at https://mta-sts.domain/.well-known/mta-sts.txt stating the MTA-STS version, mode, allowed MX host names and max_age.Topic: Security
  25. MTA-STS record:The TXT record at _mta-sts followed by the domain, v=STSv1 plus an id, telling senders a policy exists and when it has changed.Topic: Security
  26. MUA:A Mail User Agent: the program a person uses to write, send and read email, such as Outlook, Apple Mail, Thunderbird or Gmail.Topic: SMTP
  27. multipart/alternative:A MIME structure holding several versions of the same content, typically plain text and HTML, from which the mail program displays the best one it can.Topic: Message format
  28. multipart/mixed:The MIME type for a message whose parts are independent and kept in a set order, most often a message body followed by attachments.Topic: Message format
  29. multipart/related:The MIME type (RFC 2387) for parts meant to be used together, such as an HTML body and the images it embeds through cid: references.Topic: Message format
  30. MX host:A server named in a domain's MX records as a place to deliver that domain's mail; senders try the host with the lowest preference value first.Topic: SMTP
  31. MX record:The DNS record that names the servers accepting mail for a domain, each with a preference number; senders try the lowest number first.Topic: DNS

N

8 terms
  1. NDR:A non-delivery report: the message a mail system sends back to the sender to say a message could not be delivered, with the reason and usually an error code.Topic: Bounces
  2. Negative caching TTL:How long resolvers may cache the answer that a name or record does not exist; RFC 2308 sets it from the zone's SOA record.Topic: DNS
  3. NODATA:A DNS reply with response code NOERROR but no records in the answer: the name exists, but has no record of the type requested.Topic: DNS
  4. np= tag:The DMARC tag setting the policy for subdomains that do not exist in DNS, which attackers can otherwise invent to send spoofed mail.Topic: Authentication
  5. NS record:The DNS record listing the authoritative nameservers for a zone; set at the registrar, it decides which DNS host's records the world sees.Topic: DNS
  6. Null MX:A single MX record with preference 0 and the target "." that declares a domain accepts no email (RFC 7505).Topic: DNS
  7. Null sender:An empty envelope sender, written MAIL FROM:<>, used for bounces and other notifications so that they can never trigger a bounce of their own.Topic: Bounces
  8. NXDOMAIN:The DNS response code (RCODE 3) meaning the queried name does not exist at all, with no records of any type.Topic: DNS

O

11 terms
  1. One-click unsubscribe:Unsubscribing with a single action in the mail program, implemented with the List-Unsubscribe and List-Unsubscribe-Post headers of RFC 8058; Gmail and Yahoo require it of bulk senders.Topic: Message format
  2. Open rate:The share of delivered emails recorded as opened, measured with a tracking image that loads when the message is displayed, and distorted by privacy features and scanners.Topic: Reputation
  3. Open relay:A mail server that accepts mail from anyone and relays it to anyone, letting strangers send spam through it; such servers are blocked by receivers.Topic: SMTP
  4. Open tracking:Measuring email opens by counting requests for a tracking pixel, a method that privacy features and image blocking have made unreliable.Topic: Message format
  5. Opportunistic TLS:Using TLS when the other side offers it and falling back to plaintext when it does not, usually without verifying the certificate: the default between mail servers.Topic: Security
  6. Opt-in:A recipient actively indicating that they want messages from a sender, such as by ticking an unchecked box or submitting a signup form.Topic: Sending practice
  7. Opt-out deadline:The time a sender has to stop mailing someone who unsubscribed: 10 business days under CAN-SPAM and CASL; Google expects 48 hours of bulk senders and Yahoo requires 2 days.Topic: Compliance
  8. Organizational domain:The highest domain in a name's hierarchy still run by the same organization, such as example.com for mail.example.com; DMARC uses it for relaxed alignment and policy lookup.Topic: Authentication
  9. Out-of-office reply:An automatic reply telling senders that the recipient is away, set up in the recipient's mail program; it shows the message reached a mailbox.Topic: Message format
  10. Outlook high-volume sender requirements:Microsoft's rules, enforced from May 5, 2025, for domains sending over 5,000 messages a day to Outlook.com, hotmail.com and live.com: SPF, DKIM and DMARC.Topic: Providers
  11. Outlook.com:Microsoft's consumer email service, covering outlook.com, hotmail.com and live.com addresses, with its own postmaster site, SNDS data and JMRP feedback loop.Topic: Providers

P

25 terms
  1. p=none:The DMARC policy that asks receivers to take no special action on failing mail; it collects reports while the owner fixes authentication.Topic: Authentication
  2. p=quarantine:The DMARC policy asking receivers to treat mail that fails DMARC as suspicious; Google's guide, for example, describes delivery to the spam folder.Topic: Authentication
  3. p=reject:The strictest DMARC policy: the domain owner considers any failure a clear sign the use of its domain is not valid, and asks receivers to refuse the message.Topic: Authentication
  4. pct tag:A DMARC tag from RFC 7489 asking receivers to apply the policy to only a percentage of failing mail; RFC 9989 retired it as historic.Topic: Authentication
  5. PECR:The UK's Privacy and Electronic Communications Regulations, enforced by the ICO. Regulation 22 restricts marketing email to individuals but allows it to corporate subscribers.Topic: Compliance
  6. Permission pass:One email to an existing list asking every recipient to confirm they want to stay subscribed, keeping only those who click to confirm and deleting the rest.Topic: Sending practice
  7. Personalization:Tailoring each message to its recipient, from inserting a first name or company with merge tags to writing for the person's situation.Topic: Sending practice
  8. Phishing:A scam message that poses as a trustworthy organization or person to get the reader to click a malicious link, reveal credentials or send money.Topic: Security
  9. Physical postal address:CAN-SPAM's requirement that every commercial email include the sender's valid physical postal address: a street address, a registered PO box or a registered private mailbox.Topic: Compliance
  10. PIPELINING:An SMTP extension (RFC 2920) that lets a client send several commands in one batch without waiting for each reply, saving round trips.Topic: SMTP
  11. Plain-text part:The text/plain version of a message, sent alongside the HTML in a multipart/alternative structure for readers and programs that do not display HTML.Topic: Message format
  12. Port 25:The TCP port for SMTP between mail servers, used whenever one server delivers to another, for example from a sending platform to the recipient's MX host.Topic: SMTP
  13. Port 465:The TCP port for message submission over implicit TLS (RFC 8314): the TLS handshake starts as soon as the connection opens, before any SMTP command.Topic: SMTP
  14. Port 587:The TCP port for message submission (RFC 6409), where mail programs log in to their provider's server, normally upgrading the connection to TLS with STARTTLS.Topic: SMTP
  15. Postmaster address:The reserved postmaster mailbox that every domain receiving mail must accept (RFC 5321), so that problems with its mail system can always be reported.Topic: Bounces
  16. Precedence header:A non-standard header field with values such as bulk, list or junk, long used to mark mass mail so that automatic responders stay quiet.Topic: Message format
  17. Preference center:A page where subscribers choose which lists, topics or frequency they receive from a sender, alongside the option to stop all mail.Topic: Sending practice
  18. Preheader:The short line of text an inbox shows after the subject line, taken from the start of the email body; senders often set it with a hidden element at the top of the HTML.Topic: Message format
  19. Pristine spam trap:A spam trap address that was never used by a person, so it can only be on a list through harvesting, guessing or a purchased list.Topic: Reputation
  20. psd= tag:The DMARC tag from RFC 9989 that says whether a record belongs to a public suffix domain (y), an organizational domain (n), or is undetermined (u).Topic: Authentication
  21. ptr mechanism:The SPF mechanism that matches when the sending IP address has a verified reverse DNS name inside a given domain; RFC 7208 says not to use it.Topic: Authentication
  22. PTR record:The DNS record that maps an IP address back to a host name, published in the in-addr.arpa or ip6.arpa reverse zone by whoever controls the address.Topic: DNS
  23. Public suffix domain:A domain under which independent organizations register their own names, such as com, org or co.uk; DMARC lets its operator publish a policy.Topic: DNS
  24. Public Suffix List:A community-maintained list of domain suffixes under which anyone can register names, such as com, co.uk or github.io, used to find where a registrable domain begins.Topic: DNS
  25. Purchased list:A list of email addresses bought or rented from a third party. The people on it gave no permission to the buyer, and providers forbid mailing it.Topic: Sending practice

Q

2 terms
  1. QUIT command:The SMTP command that ends a session: the server replies 221 and closes the connection.Topic: SMTP
  2. Quoted-printable:A MIME transfer encoding for mostly readable text that leaves plain ASCII alone and writes other bytes as = plus two hex digits, such as =C3=A9 for é in UTF-8.Topic: Message format

R

26 terms
  1. Rate limiting:A receiving provider limiting how fast it accepts mail from a sender, answering excess connections or messages with temporary 4xx replies so they arrive later.Topic: SMTP
  2. RBL:Short for Real-time Blackhole List, the first widely used IP blocklist (1997). The name stuck and RBL is still used to mean any DNS blocklist.Topic: Reputation
  3. RCPT TO:The SMTP command that names one recipient of the message being sent; the server accepts or refuses each recipient separately.Topic: SMTP
  4. Re-engagement campaign:A message or short series sent to inactive subscribers asking whether they still want your mail, before the unresponsive ones are removed.Topic: Sending practice
  5. Read receipt:A notice sent back to the sender when a message is opened, requested with the Disposition-Notification-To header (RFC 8098) and sent only if the recipient's program agrees.Topic: Message format
  6. Received header:A trace header each mail server adds at the top of a message as it passes through, recording where it came from, which server received it, and when.Topic: Message format
  7. Received-SPF:A trace header a receiving server adds to record the SPF result it reached for a message, with the IP address and identity it checked.Topic: Authentication
  8. Recipient limit:The most recipients a server accepts in one SMTP transaction; RFC 5321 requires at least 100 and gives 452 as the reply for going over a server's own limit.Topic: SMTP
  9. Recycled spam trap:A once-real address or domain that was abandoned, then turned into a spam trap after a period of inactivity. Hitting one means old addresses were never removed.Topic: Reputation
  10. redirect modifier:The SPF modifier redirect=domain, which hands the whole check to another domain's record when nothing in the current record matched.Topic: Authentication
  11. References header:The header field in a reply listing the Message-IDs of earlier messages in the conversation, used by mail programs to rebuild a thread.Topic: Message format
  12. REFUSED:The DNS response code (RCODE 5) meaning the server declined to answer for policy reasons, such as a query for a zone it does not serve.Topic: DNS
  13. Relaxed alignment:The default DMARC alignment mode, in which the authenticated domain and the From domain only need to share the same organizational domain.Topic: Authentication
  14. Reply rate:The share of contacts who replied at least once to the emails in a campaign or sequence, counted per contact rather than per message.Topic: Reputation
  15. Reply-To header:The header field giving the address replies should go to when that is not the From address; mail programs use it when the reader clicks Reply.Topic: Message format
  16. Report spam button:The control in a mail client that files a message as spam and, at providers that run feedback loops, turns it into a complaint the sender can receive.Topic: Reputation
  17. Reputation recovery:Restoring a damaged IP or domain reputation by removing the cause, sending only to engaged recipients at steady volume, and giving providers time to see the change.Topic: Reputation
  18. REQUIRETLS:An SMTP extension (RFC 8689) by which a sender requires that one message travel only over validated TLS on every hop, or bounce.Topic: Security
  19. Retry schedule:The timing a sending server follows to re-attempt a temporarily failed delivery, from the first retry to the moment it gives up and bounces the message.Topic: SMTP
  20. Return-Path header:The header the delivering mail server adds at final delivery to record the envelope sender, the address bounces for the message go to.Topic: Message format
  21. Reverse DNS:Looking up the host name for an IP address through its PTR record, the reverse of the usual name-to-address lookup.Topic: DNS
  22. Right to object:The GDPR right, in Article 21, to stop one's personal data being used for direct marketing at any time. For marketing the right is absolute: the sender must comply.Topic: Compliance
  23. Role address:An address for a function rather than a person, such as info@, sales@, abuse@ or postmaster@, often read by several people or by software.Topic: Sending practice
  24. RSET:The SMTP command that abandons the current mail transaction, discarding sender, recipients and data, without closing the connection.Topic: SMTP
  25. rua tag:The DMARC tag listing the addresses, usually mailto: URIs, where receivers should send aggregate reports for the domain.Topic: Authentication
  26. ruf tag:The DMARC tag listing addresses for failure reports, the per-message reports a receiver may send when a message fails DMARC.Topic: Authentication

S

77 terms
  1. Scraped list:A list of addresses collected automatically from websites and directories without the owners' knowledge. Harvesting is illegal under several anti-spam laws.Topic: Sending practice
  2. Secondary sending domain:A separate domain, often resembling the brand's main one (example-mail.com for example.com), that cold emailers send from so complaints and filtering fall on it instead.Topic: Sending practice
  3. Secure email gateway:A filtering service placed in front of an organisation's mailboxes, usually by pointing its MX records at the gateway, which scans mail before passing it on.Topic: Providers
  4. Seed list:A set of test mailboxes at different providers that a message is sent to, so a testing service can check which folder each copy reached.Topic: Reputation
  5. Send throttling:A sender deliberately limiting how fast its servers deliver mail to each destination, so the pace stays within what each mailbox provider accepts.Topic: Sending practice
  6. Send-time optimization:A platform feature that picks the hour to send a campaign, or each message, from past engagement data, instead of a fixed time chosen by the sender.Topic: Sending practice
  7. Sender header:The header field naming the mailbox that actually sent a message on the author's behalf, when that differs from the From address.Topic: Message format
  8. Sender ID:A Microsoft-led sender authentication method from 2006 that checked an SPF-style record against a header address; its RFCs are now Historic.Topic: Authentication
  9. Sender reputation:The trust mailbox providers place in a sender, built from the history of mail sent from its IP addresses and domains and used to decide acceptance and placement.Topic: Reputation
  10. Sender Rewriting Scheme:A technique by which a forwarding server rewrites the envelope sender to an address in its own domain, so forwarded mail passes SPF at the next hop.Topic: Authentication
  11. Sender Score:A free 0 to 100 reputation score for sending IP addresses, created by Return Path and now offered by Validity, built from that company's own data network.Topic: Reputation
  12. Sending domain:The domain your mail is sent and authenticated from, mainly the visible From domain and the DKIM signing domain, on which providers build domain reputation.Topic: Sending practice
  13. Sending limits:The caps a mailbox or platform puts on how much one account can send: messages per day, recipients per message and recipients per day.Topic: Sending practice
  14. Sending volume:How much mail a sender sends, per day and per provider. Providers judge changes in volume as much as the level, so steady growth matters.Topic: Sending practice
  15. SERVFAIL:The DNS response code (RCODE 2) meaning the server could not process the query because of a problem on its side or further upstream.Topic: DNS
  16. Shared IP:A sending IP address used by many senders at once, usually an email service provider's customers, whose combined behaviour makes up its reputation.Topic: Sending practice
  17. Single opt-in:A signup in which an address is added to the list as soon as it is submitted, without confirming that its owner made the request.Topic: Sending practice
  18. SIZE extension:An SMTP extension (RFC 1870) through which a server announces its maximum message size and a client declares the size of a message before sending it.Topic: SMTP
  19. Smarthost:A relay server that a mail system is configured to send all its outbound mail through, instead of delivering directly to each recipient's MX host.Topic: SMTP
  20. SMTP:Simple Mail Transfer Protocol: the text conversation of commands and three-digit replies over TCP that mail servers use to pass a message from one to the next, defined by RFC 5321.Topic: SMTP
  21. SMTP AUTH:The SMTP extension (RFC 4954) through which a client logs in to a server, with a password or a token, before sending mail through it.Topic: SMTP
  22. SMTP banner:The first line a mail server sends when a client connects: a 220 reply naming the server, sent before any command is exchanged.Topic: SMTP
  23. SMTP envelope:The addresses exchanged in SMTP commands, the sender in MAIL FROM and the recipients in RCPT TO, kept separate from the From and To headers inside the message.Topic: SMTP
  24. SMTP extension:An optional SMTP capability, registered with IANA, that a server announces in its EHLO reply and a client may then use, such as SIZE, PIPELINING or SMTPUTF8.Topic: SMTP
  25. SMTP rejection:A receiving server refusing a message during the SMTP session with a 5xx reply, so the sender knows at once and its own system writes the bounce.Topic: Bounces
  26. SMTP relay service:A hosted service that accepts mail from your applications, devices or servers over SMTP and delivers it from the provider's infrastructure.Topic: SMTP
  27. SMTP reply code:The three-digit number a mail server returns for every SMTP command, such as 250, 421 or 550, whose first digit says whether the step succeeded, failed for now or failed for good.Topic: SMTP
  28. SMTP session:One SMTP connection from the server greeting to QUIT, during which a client identifies itself, may authenticate and start TLS, and sends one or more messages.Topic: SMTP
  29. SMTP smuggling:An email spoofing attack published by SEC Consult in December 2023 that hides a second message inside the first by exploiting servers that disagree about where message data ends.Topic: SMTP
  30. SMTP timeout:How long each side of an SMTP session waits for the other; RFC 5321 sets minimums per command, such as five minutes for the greeting and ten for the reply after the data.Topic: SMTP
  31. SMTP transaction:The part of an SMTP session that delivers one message: a MAIL FROM command, one RCPT TO per recipient, then DATA and the message itself.Topic: SMTP
  32. SMTPUTF8:An SMTP extension (RFC 6531) that allows UTF-8 in email addresses and header fields, needed for addresses written in non-Latin scripts or with accented local parts.Topic: SMTP
  33. SNDS:Outlook.com's Smart Network Data Services: a portal where IP owners see data about the mail their addresses send to Outlook.com and manage the JMRP feedback loop.Topic: Providers
  34. Snowshoe spam:Unsolicited bulk email spread across many IP addresses and domains, each sending a small volume, so no single address draws enough attention to be filtered.Topic: Reputation
  35. SOA record:The start-of-authority record at the top of every DNS zone, holding the primary nameserver, the administrator's address, a serial number and timing values.Topic: DNS
  36. Soft bounce:A delivery failure that may clear up on its own, such as a full mailbox or a temporarily unavailable server, as opposed to an address that does not exist.Topic: Bounces
  37. Soft opt-in:The EU and UK exception that lets a business email its own customers about similar products without fresh consent, if they could opt out at collection and in every message.Topic: Compliance
  38. sp= tag:The DMARC tag setting the policy for existing subdomains of the organizational domain, when it should differ from the domain's own p= policy.Topic: Authentication
  39. Spam:Unsolicited bulk email: mail sent to many recipients who did not give verifiable permission, whatever its content. Laws and providers each draw the line differently.Topic: Reputation
  40. Spam complaint:A recipient's report that a message is unwanted, usually made with the mail client's spam button, which providers count against the sender.Topic: Reputation
  41. Spam confidence level:Microsoft's spam rating, -1 or 0 to 9, stamped in the X-Forefront-Antispam-Report header. In the cloud it no longer decides the verdict on its own.Topic: Providers
  42. Spam filter:Software at the receiving side that classifies incoming mail as wanted, spam or worse, then accepts, rejects, quarantines or files it in the spam folder.Topic: Reputation
  43. Spam folder:The folder where a mailbox provider files accepted messages it judges unwanted: Spam in Gmail, the Junk Email folder in Microsoft mail, outside the main inbox.Topic: Reputation
  44. Spam score:The number a filter assigns to a message from the tests it matched, compared with a threshold to decide whether the message is spam. Each filter has its own scale.Topic: Reputation
  45. Spam trap:An email address used to collect and monitor unsolicited mail. It never signs up for anything, so any mail it receives shows how the sender got the address.Topic: Reputation
  46. SpamAssassin:The open-source spam filter of the Apache Software Foundation, which adds up scores from many tests and marks a message as spam above a threshold, 5 by default.Topic: Reputation
  47. SpamCop:A spam reporting service whose SpamCop Blocking List (bl.spamcop.net) lists IP addresses that sent mail its users reported, and drops them once reports stop.Topic: Reputation
  48. Spamhaus:A blocklist operator whose IP lists (combined as ZEN) and Domain Blocklist are queried by mail servers to reject or score spam.Topic: Reputation
  49. Spear phishing:Phishing aimed at a specific person or organization, using details about them to make the message convincing; aimed at executives it is called whaling.Topic: Security
  50. SPF:Sender Policy Framework: a DNS record in which a domain lists the servers allowed to send mail for it, so a receiver can check the connecting IP address.Topic: Authentication
  51. SPF "a" mechanism:The SPF mechanism that matches when the sending IP address is one of the A or AAAA addresses of the current domain or of a named host.Topic: Authentication
  52. SPF "all" mechanism:The SPF mechanism that always matches; placed last in a record, its qualifier sets the result for every address the earlier terms did not authorize.Topic: Authentication
  53. SPF "mx" mechanism:The SPF mechanism that matches when the sending IP address belongs to one of the mail exchangers listed in a domain's MX records.Topic: Authentication
  54. SPF alignment:The DMARC condition met when SPF passes for a MAIL FROM domain that matches the From domain, by organizational domain or exactly.Topic: Authentication
  55. SPF domain:The domain whose SPF record is checked: the domain of the SMTP MAIL FROM address, or the HELO name when that address is empty.Topic: Authentication
  56. SPF fail:The SPF result stating explicitly that the sending IP address is not authorized to use the domain, produced by a - qualifier such as -all.Topic: Authentication
  57. SPF flattening:Rewriting an SPF record so that includes are replaced by the IP addresses they resolve to, bringing the record under the ten-lookup limit.Topic: Authentication
  58. SPF lookup limit:The rule that one SPF check may involve at most ten mechanisms or modifiers that need DNS queries; exceeding it returns permerror.Topic: Authentication
  59. SPF macro:A placeholder such as %{i} or %{d} in an SPF record that the receiver replaces with a value from the SMTP session before doing the lookup.Topic: Authentication
  60. SPF neutral:The SPF result when the domain's record explicitly makes no assertion about the sending IP address, usually from a ?all ending.Topic: Authentication
  61. SPF none:The SPF result when the domain publishes no SPF record, or no valid domain could be taken from the session to check.Topic: Authentication
  62. SPF pass:The SPF result stating that the sending IP address is authorized to send mail for the domain in the checked identity.Topic: Authentication
  63. SPF permerror:The SPF result when the domain's published record cannot be interpreted, such as two SPF records or more than ten lookups; only a DNS fix resolves it.Topic: Authentication
  64. SPF qualifier:The one-character prefix on an SPF mechanism that sets the result when it matches: + for pass, - for fail, ~ for softfail, ? for neutral.Topic: Authentication
  65. SPF record:The TXT record, starting with v=spf1, that holds a domain's SPF policy: an ordered list of mechanisms and modifiers ending in a default such as -all.Topic: Authentication
  66. SPF softfail:The SPF result saying the sending IP address is probably not authorized, a weaker statement than fail, produced by a ~ qualifier such as ~all.Topic: Authentication
  67. SPF temperror:The SPF result when a temporary error, usually a DNS timeout or server failure, stopped the check; a later retry may succeed unchanged.Topic: Authentication
  68. Spintax:A template syntax listing alternatives in braces, such as {Hi|Hello}, from which a sending tool picks one per message so each recipient gets a slightly different text.Topic: Sending practice
  69. STARTTLS:The SMTP command that upgrades an open plaintext connection to TLS, after the server advertises support in its EHLO response.Topic: Security
  70. Strict alignment:The DMARC alignment mode in which the authenticated domain must be identical to the From domain, selected with adkim=s or aspf=s.Topic: Authentication
  71. Subaddressing:Adding a tag to the local part of an address, usually after a plus sign (ana+news@example.com), so mail reaches the same mailbox but can be sorted by tag.Topic: Message format
  72. Subdomain:A domain inside another, such as mail.example.com within example.com; it can carry its own DNS records, including separate SPF, DKIM and DMARC.Topic: DNS
  73. Subject header:The header field holding the message's topic line, shown in the inbox next to the sender name and preview text.Topic: Message format
  74. Sunset policy:A rule that stops mailing subscribers who have not engaged for a set period, often after a last re-engagement attempt, to protect reputation and keep lists current.Topic: Sending practice
  75. Suppression list:A list of addresses that must never be mailed again (unsubscribes, complainants, hard bounces), checked before every send whatever list the address appears on.Topic: Sending practice
  76. SURBL:A reputation data provider whose best-known list, Multi, lists domains of malicious or abused websites so filters can judge messages by the links they contain.Topic: Reputation
  77. SVG Tiny PS:The restricted SVG profile BIMI logos must use: SVG Tiny 1.2 with baseProfile tiny-ps, a title, and no scripts, animation or external references.Topic: Authentication

T

14 terms
  1. Tarpitting:A defence in which a mail server deliberately delays its SMTP replies to clients it suspects of abuse, making bulk probing and dictionary attacks slow and expensive.Topic: SMTP
  2. TLS:Transport Layer Security: the protocol that encrypts and authenticates a connection, used in email between servers through STARTTLS and between apps and servers.Topic: Security
  3. TLS certificate:A signed file binding a public key to one or more host names, which a server presents during the TLS handshake to prove its identity.Topic: Security
  4. TLS-RPT:SMTP TLS Reporting: a TXT record at _smtp._tls naming where sending servers should send daily reports of TLS connection successes and failures to a domain.Topic: Security
  5. TLSA record:The DNS record used by DANE that ties a TLS certificate or public key to a service, published at a name such as _25._tcp.mx.example.com.Topic: DNS
  6. To header:The header field listing the message's primary recipients as the author addressed them; delivery itself follows the envelope, not this field.Topic: Message format
  7. Top-level domain:The last label of a domain name, such as com, org or fr: a zone one level below the DNS root, run by a registry.Topic: DNS
  8. Tracking domain:The domain that tracked links and open pixels in your emails point to. A custom one on your own domain keeps link reputation separate from other senders.Topic: Sending practice
  9. Tracking pixel:A tiny, usually invisible image in an HTML email whose unique URL tells the sender's server when, and from where, the message was opened.Topic: Message format
  10. Transactional email:Email that completes or reports on something the recipient did or has with you: receipts, password resets, account alerts. Its reputation is worth keeping apart from marketing.Topic: Sending practice
  11. Trusted ARC sealer:An intermediary whose ARC results a receiver has chosen to believe, so mail it forwards can be accepted even when SPF, DKIM or DMARC now fail.Topic: Authentication
  12. TTL:Time to live: the number of seconds a DNS answer may be cached by resolvers before they must ask the authoritative server again.Topic: DNS
  13. TXT record:A DNS record holding free-form text, used to publish SPF, DKIM keys, DMARC, MTA-STS, TLS-RPT, BIMI and domain verification tokens.Topic: DNS
  14. Typo trap:A spam trap built on a common misspelling, usually of a domain such as gmial.com, which catches senders who never confirm the addresses they collect.Topic: Reputation

U

5 terms
  1. Unsubscribe:The way a recipient stops future mail from a sender: a link in the message, the List-Unsubscribe header or a reply. Laws and providers set how fast it must work.Topic: Sending practice
  2. Unsubscribe rate:The share of recipients who unsubscribed after a send. An easy unsubscribe keeps people who no longer want the mail from reporting it as spam instead.Topic: Reputation
  3. URI blocklist:A blocklist of domains found in message bodies, mainly link domains. Filters look up each link and score or block the message, whatever IP sent it.Topic: Reputation
  4. URIBL:A domain reputation list, URIBL.COM, that lists domains seen in unsolicited mail. Filters such as SpamAssassin check message links against multi.uribl.com.Topic: Reputation
  5. User unknown:A delivery failure because the recipient address does not exist at the destination domain, reported with reply code 550 and enhanced status code 5.1.1.Topic: Bounces

V

4 terms
  1. Verified Mark Certificate:A BIMI mark certificate for a registered trademark or government mark, issued after the certification authority verifies the organization and the trademark.Topic: Authentication
  2. VERP:Variable Envelope Return Path: giving each recipient's copy its own envelope sender that encodes the recipient, so any bounce identifies the failed address without parsing.Topic: Bounces
  3. void lookup:A DNS query made during an SPF check that returns no answer (NXDOMAIN or an empty NODATA reply); receivers should allow at most two.Topic: Authentication
  4. VRFY:An SMTP command that asks a server whether an address is a real mailbox; most public servers disable it or give a non-committal 252 reply.Topic: SMTP

W

3 terms
  1. Warm-up service:A service whose network of mailboxes exchanges messages with yours, replies to some and moves any found in spam to the inbox, to generate engagement before real sending.Topic: Sending practice
  2. Webmail:Email read and written in a web browser, such as Gmail or Outlook on the web, where the mail program's work happens on the provider's servers.Topic: Message format
  3. Wildcard DNS record:A DNS record whose leftmost label is *, answering for names that do not otherwise exist under that point in the zone.Topic: DNS

X

1 term
  1. X-header:A non-standard header field whose name starts with X-, such as X-Mailer, added by software for its own purposes; RFC 6648 discourages the prefix for new fields.Topic: Message format

Y

4 terms
  1. Yahoo Complaint Feedback Loop:Yahoo's feedback loop: senders who enroll a DKIM signing domain receive a report each time a Yahoo or AOL recipient marks one of their messages as spam.Topic: Providers
  2. Yahoo Mail:Yahoo's consumer email service, which also hosts AOL Mail. One set of sender requirements, one Sender Hub and one feedback loop cover both brands.Topic: Providers
  3. Yahoo Sender Hub:Yahoo's site for senders to Yahoo and AOL addresses: its sender requirements, the Complaint Feedback Loop, error codes and a dashboard with delivery statistics per domain.Topic: Providers
  4. Yahoo sender requirements:Yahoo's rules for mail to Yahoo and AOL users: SPF or DKIM and a spam rate under 0.3% for everyone, plus DMARC, alignment and easy unsubscribe for bulk senders.Topic: Providers

Z

1 term
  1. Zone apex:The top name of a DNS zone, such as example.com itself rather than www.example.com, where the zone's SOA and NS records live.Topic: DNS

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