Within the Internet
email
Electronic mail (usually shortened to email; alternatively hyphenated e-mail) is a method of transmitting and receiving Digital media, digital messages using electronics, electronic devices over a computer network. It was conceived in the ...
system, a message transfer agent (MTA), mail transfer agent, or mail relay is
software
Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications.
The history of software is closely tied to the development of digital comput ...
that transfers electronic mail messages from one computer to another using the
Simple Mail Transfer Protocol. 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
mail submission agent (MSA), or a
mail user agent
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal sy ...
(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 trace header field to the top of the header of the message, thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.
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. this distinction is clarified in .
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 field is added to the envelope to record the
return path.
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
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. Today, POP version 3 (POP3) is the most commonly used version. Together with IMAP, ...
(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
ISP. This is for policy, not technical, reasons so that providers have some means of holding their users accountable for the generation of
spam
Spam most often refers to:
* Spam (food), a consumer brand product of canned processed pork of the Hormel Foods Corporation
* Spamming, unsolicited or undesired electronic messages
** Email spam, unsolicited, undesired, or illegal email messages
...
and other forms of email abuse.
See also
*
List of mail server software
*
Relay (disambiguation)
References
{{DEFAULTSORT:Message Transfer Agent
Internet mail protocols
Email agents