A User Agent (UA) provides the the interface between the user and the Message Transfer System (MTS). The MTS consists of some number of Message Transfer Agents (MTAs) collectively charged with the responsibility of moving messages from the originator to the recipient.
The UA is the interface between the e-mail system and its user. Examples of popular UAs Microsoft Outlook, Microsoft Exchange, Lotus, Eudora, and Thunderbird. Much of the UA is concerned with matters not affecting communication and is thus part of the application process that is strictly local and not regulated by communications standards. The UA provides the ability to: execute mail system operation, compose or access the message that is to be sent, know about and retreive incoming messges, provide the destination description the mail system requires to deliver the message, and present incoming messages to the user.