MUA (Mail User Agent)

A MUA (Mail User Agent) is the application at the user's end of the mail system: it lets you write messages, hands them to a server for sending and fetches or displays the mail waiting in your mailbox. Another everyday word for it is an email client.

Why it matters

The term separates the human-facing program from the server-side components. A MUA does not route mail between domains; it submits outgoing mail to an MTA using SMTP and reads stored mail using IMAP, POP3 or a proprietary protocol. Knowing this helps when troubleshooting, because a problem can sit in the client (wrong settings, a broken rule), in transit, or in the server.

Example

When you press Send in Thunderbird:

1. The MUA connects to smtp.example.com on port 587 and authenticates.
2. It submits the message; from there MTAs handle delivery.
3. Later, it uses IMAP to show you replies from the mailbox.

Switching from Thunderbird to Apple Mail changes only the MUA; the mailbox and server are untouched.

Common mistakes

• Blaming the server when the same mailbox works in webmail but not in the desktop client; the client settings are the likely cause.
• Confusing the MUA with the mail provider; Gmail is both a service and a webmail MUA.
• Forgetting that client-side rules and filters can hide mail that did arrive.

Related

• MTA
• MDA
• Email Client
• Webmail

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.