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Error code wiki · 17 codes

IONOS SMTP error codes

IONOS, formerly 1&1, hosts email for the domains of its customers, so mail to an address on an IONOS-hosted domain is received and filtered by IONOS mail servers.

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Permanent failures (5xx)

13 codes

The receiving server refused the message for good. Your server will not try again, and the message comes back to you as a bounce.

  1. 535SMTP authentication failed.Your email program could not sign in to the IONOS outgoing server because the username or password stored in it is wrong.Permanent
  2. 550Mailbox unavailable.The IONOS-hosted address you wrote to no longer exists, or the mailbox was temporarily deactivated after a long period without use.Permanent
  3. 550Spam domain or invalid HELO.IONOS refused the mail because your sender domain is known as a spam domain, or because your server HELO/EHLO is not a fully qualified domain name.Permanent
  4. 550Administrative reject.IONOS does not allow delivery from your system for administrative reasons. Only IONOS can say why, so the step is to contact it.Permanent
  5. 550Sender domain has no valid MX or A record.IONOS refused the mail because your sender domain is not set up in DNS as the RFCs require, so a reply to it could not be delivered.Permanent
  6. 550Sender domain publishes a null MX.Your sender domain publishes a No Service MX record (null MX), which says it accepts no mail, and IONOS rejects mail from such domains.Permanent
  7. 552Recipient mailbox is full.The recipient's IONOS mailbox has no free space left, so the message could not be delivered. Only the recipient can free space.Permanent
  8. 554No reverse DNS for your IP.IONOS refused your mail because the IP address of your mail server has no domain name assigned to it in DNS (no PTR record).Permanent
  9. 554Reverse DNS does not meet IONOS policy.IONOS refused your mail because the PTR record of your IP does not meet its policy, typically a generic or dynamic-looking provider name.Permanent
  10. 554IP address on a DNS blocklist.IONOS refused your mail because your sending IP has an entry on a DNS blocklist (DNSBL). IONOS will check an entry you believe is wrong.Permanent
  11. 554Rejected under IONOS policy.IONOS refused the mail because it violates IONOS policy, most often because the sending server is mainly sending spam.Permanent
  12. 554Headers break RFC 5322.IONOS refused the message because its headers do not follow RFC 5321 and RFC 5322: a missing or doubled Date, or a From, Sender, To, Cc or Subject header that is repeated or malformed.Permanent
  13. 556Recipient domain publishes a null MX.The domain you are writing to has a No Service MX record (null MX), which means it can neither receive nor send email, so the message cannot be delivered.Permanent

Temporary failures (4xx)

4 codes

The receiving server cannot take the message now. Your server keeps it and retries, and only bounces it if the retries run out.

  1. 421IP blocked for mail to unknown addresses.IONOS has blocked your server IP from sending to its customers for now, because too much of your mail went to addresses that do not exist.Temporary
  2. 421Rate limit reached.Your server sent too many emails to IONOS in too short a time, so IONOS deferred the rest. Send fewer at once and try again later.Temporary
  3. 450Deferred for a changing HELO or unknown addresses.IONOS delayed your mail because your server keeps changing its HELO/EHLO name, or because it sent too much to addresses that do not exist.Temporary
  4. 451Local error in processing.IONOS could not accept the message because the service was temporarily unavailable. IONOS points to slow DNS answers from the sending side as the thing to check.Temporary

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