Mail transfer agent

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A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent that 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 an MSA itself. The MTA works behind the scenes, while the user usually interacts with the MUA.

The delivery of e-mail 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.