(MTA, Mail Transfer Agent) Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA, often by SMTP over the Internet, it stores it temporarily locally and analyses the recipients and delivers it to any local addressees and/or forwards it to other remote MTAs (routing) for delivery to remote recipients. In either case it may edit and/or add to the message headers.
The most widely used MTA for Unix is sendmail, which communicates using SMTP.
RFC 2821 (SMTP) expands MTA as "Mail Transfer Agent" though this is less common. Alternatives with "Transport" are also seen but less correct.