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Mail Server
A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. The MTA works behind the scenes, while the user usually interacts with another program, the mail user agent (MUA), which contacts an MTA for actual delivery of the mail.
The delivery of email to a user's mailbox typically takes place via a mail delivery agent (MDA); many MTAs have basic MDA functionality built in, but a dedicated MDA like procmail can provide more sophistication.
The sendmail program, which implements the standard SMTP mail transfer protocol, has become a very widely used MTA. The Microsoft Exchange Server contains an MTA as well. Other MTAs include qmail, Postfix, Exim, Courier and ZMailer.
See also
MTA as an abbreviation can have other meanings: see the MTA disambiguation page.