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.
What it means
RFC 5321 defines a relay as a system that takes mail from an SMTP client and transmits it to another SMTP server, without changing the message beyond adding trace information. Every MTA between the sender's submission server and the recipient's mailbox server is a relay, and each adds a Received header.
Relaying is a permission. A well-run server relays only for its own authenticated users or trusted networks and, from everyone else, accepts only mail addressed to the domains it hosts; refusing other relay attempts with 550 is normal, as in Microsoft's 5.7.1 unable to relay. A server that relays for anyone is an open relay. Organisations also run internal relays, or use a hosted relay service, to send mail from applications and devices.
Related terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes