The terms Mail User Agent (MUA) and Message User Agent (MUA) are used interchangeably. It is an e-mail client in which the user enters messages, creates, sends, receives and reads e-mails.
Messages are transferred to the mail server using Simple Mail Transfer Protocol( SMTP) and retrieved using Post Office Protocol( POP) or Internet Message Access Protocol( IMAP).
The Mail User Agent is a service provider that makes Message Transfer Service( MTS) services available to the user. It communicates with the Mail Transfer Agent( MTA) as the user of the message services. Mail User Agents can read messages from mailboxes and also reply to them.