MTA

From Qmail Info Wiki

MTA is a generic term which means "mail transfer agent". Basically, it's a "mail server" program- software whose job is to move email from one place to another. Examples of MTA's are qmail, sendmail, postfix, and exim.

The complement of an MTA would be an MUA, or "mail user agent", which is the software that a user would run on their desktop to actually work with their email- reading mail, writing new messages, forwarding message, and so forth. Examples of this would be Evolution, Thunderbird, Eudora, and the "Mail" program included with Apple's OS X.