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{{Short description|Software to transfer electronic mail}}
{{Use dmy dates|date=
Within the Internet [[email]] system, a '''message transfer agent''' ('''MTA'''),<ref>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.</ref> '''mail transfer agent''',<ref>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.</ref> or '''mail relay''' is [[software]] that transfers electronic mail messages from one computer to another using the [[Simple Mail Transfer Protocol]].<ref>
</ref> The terms '''mail server''', '''mail exchanger''', and '''MX host''' are also used in some contexts.▼
RFC 5598, ''Internet Mail Architecture'', D. Crocker (July 2009).
▲</ref>
Messages exchanged across networks are passed between mail servers, including any attached data files (such as images, multimedia, or documents). These servers
==Operation==
A message transfer agent receives mail from either another MTA, a [[mail submission agent]] (MSA), or a [[mail user agent]] (MUA). The transmission details are specified by the [[Simple Mail Transfer Protocol]] (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a
[[File:E-mail.svg]]
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
For recipients hosted locally, the final delivery of email to a recipient mailbox is the task of a [[message delivery agent]] (MDA). For this purpose the MTA transfers the message to the message handling service component of the message delivery agent (MDA). Upon final delivery, the
==Transfer versus access==
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* 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 [[ISP]]. This is for policy, not technical, reasons so that providers have some means of holding their users accountable for the generation of [[Spam (electronic)|spam]] and other forms of email abuse.<ref>{{cite web |author=Bill Cole |title=What are the IPs that sends mail for a domain? |url=http://www.ietf.org/mail-archive/web/asrg/current/msg15593.html |date=29 June 2009 |work=[[Anti-Spam Research Group|ASRG]] mailing list |
== See also ==
*
* [[Relay (disambiguation)]]
==References==
{{
{{Authority control}}
{{DEFAULTSORT:Message Transfer Agent}}
[[Category:Internet mail protocols]]
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