MUA – Mail User Agent

Software operated by a person, or on their behalf, to recieve and transmit mail. Contrast MTA. Subclass: Message Submission Agent (MSA).

Common MUAs: Outlook, Outlook Express, Thunderbird, ,Eeudora, mailx, mozilla and many many more.

MUAs tend to use cut-down, often out-of-spec variants of SMTP for sending mail; the first MTA needs to treat them less harshly than it does mail from other MTAs.