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.

It receives messages from another MTA (relaying), a mail submission agent (MSA) that itself got the mail from a mail user agent (MUA) or directly from an MUA, thus acting as a MSA itself. The MTA works behind the scenes, while the user usually interacts with the MUA.

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.

See also

• List of mail servers

MTA as an abbreviation can have other meanings: see the MTA disambiguation page.