Question: what is the difference between user agent and mail transfer agent?
Answer: An MUA is a mail user agent - this is a program that allows a user to read, compose, and send e-mail. ... An MTA is a mail transfer agent - this is a program that is responsible for the transport, delivery, and forwarding of e-mail. SMTP servers like sendmail are MTAs. There will typically be at most one MTA on a machine.