Message transfer agent
Within the Internet email system, a message transfer agent (MTA),[1] mail transfer agent,[2] or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol.[3] In some contexts, the alternative names mail server, mail exchanger, or MX host are used to describe an MTA.
Messages exchanged across networks are passed between mail servers, including any attached data files (such as images, multimedia, or documents). These servers often keep mailboxes for email. Access to this email by end users is typically either by webmail or an email client.
Operation
A message transfer agent receives mail from either another MTA, a
An MTA works in the background, while the user usually interacts directly with a mail user agent. One may distinguish initial submission as first passing through an MSA—port 465 (or, for legacy reasons, optionally port 587) is used for communication between an MUA and an MSA, while port 25 is used for communication between MTAs, or from an MSA to an MTA.
For recipients hosted locally, the final delivery of email to a recipient mailbox is the task of a
Transfer versus access
A relay or filtering server will typically store email only briefly, but other systems keep full mailboxes for email - in which case they usually support some means for end users to access their email via a Mail User Agent (MUA), or email client.
Common protocols for this are:
- Post Office Protocol (POP3)
- Internet Message Access Protocol (IMAP)
- Proprietary systems, such as Microsoft's MAPI
Submission of new email from a mail client is via SMTP, typically on port 587 or 465, and is now generally restricted to servers the user has an account with-such as their
See also
References
- ^ MTA=Message Transfer Agent (similar to X.400 name) is found, e.g., in RFC 1506, RFC 2476, RFC 3461, RFC 3464, RFC 3865, RFC 3888, RFC 6409, RFC 5598.
- ^ MTA=Mail Transfer Agent (similar to Mail Transfer Protocol) is found, e.g., in RFC 2298, RFC 2305, RFC 3804, RFC 3798, RFC 4496, RFC 5442, RFC 5429.
- ^ RFC 5598, Internet Mail Architecture, D. Crocker (July 2009).
- ^ See Email#Message header for the format of an email message. Many MUAs allow users to see the raw message source directly, thereby allowing header inspection.
- ^ See table at Email client#Port numbers
- ^ Bill Cole (29 June 2009). "What are the IPs that sends mail for a domain?". ASRG mailing list. Retrieved 15 September 2009.