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.
What it means
Early mail servers often relayed for all comers, and spammers exploited that. RFC 2505 (BCP 30, 1999) requires that an MTA be able to restrict unauthorised use as a mail relay, and today an open relay is a misconfiguration: a server that passes mail from an outside sender to an outside recipient without authentication.
Receivers refuse mail from such servers: Outlook.com, for example, rejects with 550 SC-003 when the connecting IP appears to be an open proxy or relay. The check is simple. From outside the network, try to send from one external address to another; the server should refuse at RCPT TO.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2505: Anti-Spam Recommendations for SMTP MTAsrfc-editor.org
- Outlook.com Postmaster: Troubleshootingsubstrate.office.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes