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

image

User agents

Email users employ ...

Get UNIX System Administration Handbook, Third Edition now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.