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What is a Mail User Agent?

By R. Kayne
Updated May 17, 2024
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A mail user agent (MUA) is an email program; software designed to collect and send electronic mail. It is also referred to as an email program, or email client. The term “mail user agent” is less familiar to the average person, but is used in email headers. The headers of the email supply information to the mail servers or computers that handle transferring messages across networks like the Internet.

Networking standards have been established to make exchange of messages possible from a virtual unlimited pool of clients. To this end every mail user agent handles text messages, graphics and attached messages in a similar manner. When a user sends an email an array of information is sent with the message to ensure it reaches its destination. This extra “addressing” information is included in the headers, along with the name of the email program as the “mail user agent.”

Email sent from an Internet Service Provider (ISP) is handled by that ISPs mail server, which is the equivalent of an electronic post office. The mail comes first to the mail server, is processed, and forwarded towards the destination. Anther mail server resides at the destination. It receives all incoming mail and electronically sorts it into mailboxes. The recipient picks up email by using his or her email program to connect to the mail server which request items from the mailbox.

Another type of mail user agent is Web-based. In this case an email program is built into a website for public use. The server houses many mailbox accounts that users can access through providing a username and password. Once logged into a mailbox, one can write and send email or collect email. Each user maintains a unique email address assigned by the website for his or her exclusive use.

The advantage of a Web-based mail user client is that one can access the Web account anywhere there is an available Internet connection. This might be from a friend’s computer, cell phone, or while traveling by using a hotel or Internet café computer. The disadvantage of Web-based email is that all mail sent and received resides on a public mail server, rather than on your ISP’s private mail server. A Web-based mail server might be less secure than your ISP’s mail server, though no email is truly private unless encrypted.

Microsoft™ Outlook Express™, David Harris’s Pegasus™, and Mozilla™’s Thunderbird™ are among the most popular free mail user agents. Popular shareware email clients include Opera Mail™ (bundled with the Opera browser), The Bat! from RiTLabs, and many more.

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