UNIX System Administration Handbook, Third Edition

19.1 MAIL SYSTEMS

In theory, a mail system consists of four distinct components:

•   A “mail user agent” (MUA) that lets users read and compose mail

•   A “mail transport agent” (MTA) that routes messages among machines

•   A “delivery agent” that places messages in a local message store¹

•   An optional “access agent” that connects the user agent to the message store (e.g., IMAP or POP)

Some sites also use a mail submission agent that speaks SMTP (the mail transport protocol) and does some of the work of the transport agent. Exhibit A shows the relationship of these components.

Exhibit A Mail system components

User agents

Email users employ ...