What is MUA?
Also known as: Mail User Agent
A Mail User Agent (MUA) is an email client application that lets end users compose, send, receive, and manage messages on their own device or through a web interface.
A Mail User Agent (MUA) is the software component of the email infrastructure that an end user interacts with directly. It is responsible for composing new messages, reading incoming messages, and managing folders or labels. Common examples include Microsoft Outlook, Mozilla Thunderbird, Apple Mail, and webmail interfaces like Gmail or Outlook.com. The MUA is distinct from the Mail Transfer Agent (MTA) and the Mail Delivery Agent (MDA), which handle message routing and storage on servers.
When a user composes a message, the MUA formats it according to RFC 5322 (Internet Message Format) and submits it to an MTA using SMTP, typically on port 587 or 465. To retrieve messages, the MUA connects to an MDA or IMAP server using protocols like IMAP (port 143 or 993) or POP3 (port 110 or 995). Modern MUAs support features such as encryption via TLS, digital signatures with S/MIME or PGP, and synchronization across devices using IMAP.
The MUA sits at the edge of the email delivery chain. It does not relay messages between servers; that is the job of the MTA. Instead, it acts as the user's interface to both sending and receiving infrastructure. The distinction between MUA and MTA is fundamental to email architecture, as defined in RFC 5598 (Internet Mail Architecture). The MUA's role has expanded over time to include spam filtering, address book management, and calendar integration, but its core function remains unchanged: it is the user-facing agent in the email system.
Key facts
- MUA is the client software used by end users to compose and read email.
- It submits outgoing messages to an MTA via SMTP on port 587 or 465.
- It retrieves incoming messages from a server using IMAP (port 143/993) or POP3 (port 110/995).
- MUAs can be desktop applications, mobile apps, or webmail interfaces.
- The MUA does not relay messages between servers; that is the role of the MTA.
How it works in practice
Related terms
References
More in Email
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MDA
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