a program that delivers electronic mail to other users on the system or to other machines on the network
A Mail Transfer Agent is like a postman for e-mails. They direct your e-mail by looking at the e-mail address.
A facility that accepts messages generated by an Mail User Agent, determines a route for delivery, edits the message header as required by the destination and delivery program, and calls the appropriate delivery program to deliver the mail. SCO OpenServer systems provide two MTAs: MMDF and sendmail(ADMN).
(MTA) The program responsible for delivering e-mail messages. Upon receiving a message from a mail user agent or another MTA it stores it temporarily locally and analyzes the recipients and either delivers it (local addressee) or forwards it to another MTA. In either case it may edit and/or add to the message headers. A widely used MTA for Unix is sendmail.
A program, typically running on a Unix server, that delivers mail to another machine on an IP network.
A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger 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.