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
Email users employ ...