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.
What it means
In the Internet mail architecture (RFC 5598) the MTA relays a message one hop at a time: it accepts the message from a submission server or another MTA, picks the next hop, normally from the recipient domain's MX records, and passes it on, adding trace information as it goes. It stores what it cannot deliver yet and retries, and it is the component that finally gives up and generates a bounce.
Sending platforms run outbound MTAs; mailbox providers run inbound MTAs at the hosts their MX records name. In deliverability work, the outbound MTA is where retries, connections per destination and per-provider throttling are configured, which shapes how receivers see a sender.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5598, section 4.3.2: Message Transfer Agent (MTA)rfc-editor.org
- RFC 5321, section 2.3.3: Mail Agents and Message Storesrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes