A mail transfer agent or MTA (also commonly called mail server) 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 is typically done by a mail delivery agent (MDA); many MTAs have basic MDA functionality built in, but a dedicated MDA like procmail can be used if a more sophisticated tool is needed.
One of the most popular MTAs is sendmail, which implements the standard SMTP mail transfer protocol. The Microsoft Exchange server contains an MTA as well. Other MTAs include qmail, Postfix, Exim, Courier and ZMailer.