Guide to Unix/Explanations/Mail Server Overview/Incoming mail

Incoming mail is held in mailboxes hosted by various providers
Incoming Mail is delivered to mailboxes held by various service providers.

Collection
The mailserver will periodically run a mail retrieval agent that uses post office protocol to collect the mail messages from the users' mailboxes.

Transportation
The mail retrieval agent passes the mail to a spam filtering program.

Spam Filtering
The spam filtering program marks junk mail as spam.

Delivery
The filtered mail is forwarded to the mail delivery agent, either directly, or via a mail transport agent (depending on the capabilities of the mail retrieval agent). The mail delivery agent delivers the mail to either the spam box or the inbox the users maildir directory depending on whether or not the mail is marked as spam.

The IMAP server
The messages are now made visible to users via the imap server.