MUA

From Qmail Info Wiki

MUA is a generic term which means "mail user agent". Basically, it's the software that a user would run on their desktop to actually work with their email- reading mail, writing new messages, forwarding message, and so forth. Examples of this would be Evolution, Thunderbird, Eudora, and the "Mail" program included with Apple's OS X.

The complement would be an MTA, or "mail transfer agent", which is the software on a mail server whose job is to move email from one place to another. Examples of MTA's are qmail, sendmail, postfix, and exim.