Abbreviated ``MUA.'' Sometimes termed ``Mailbox agent.'' Refers to the end-user software which allows users to compose, address, send, receive, and read mail messages. There may be multiple MUAs available on a given platform, each of which may interoperate with multiple other MUAs via MTAs. MUAs may be very basic, allowing only text messages to be read or composed, or they may be very complex, supporting multi-media messages, etc.

The portion of the e-mail system that directly interacts with the end user; it may exist on one machine or may be divided across multiple machines. Although the MUA configuration may change, each individual MUA consists of an e-mail front-end, an e-mail server, and the sub-components of each.

A program that provides a user interface for mail message processing, including mail composition and reading. SCO OpenServer systems provide three MUAs: mail, scomail(XC), and SCO Shell Mail.

See email client.