An email is sent to your server via the SMTP protocol on TCP port 25. Postfix accepts the connection, reads the email and does some basic checks. Is the sender blacklisted on a realtime blacklist? Is the email from an authenticated user so we bypass relay checks? Or is the recipient of the email a valid user on our system? If we don’t trust the remote system yet we apply greylisting. At this stage Postfix can reject the email or accept it.
Postfix forwards the email via the SMTP protocol on the TCP port 10024 to AMaViS for content checking. Notice that at this stage the email can’t be rejected any more. So AMaVis can either accept it or throw it away. Commonly AMaViS is configured to add a certain email header so the user can see that AMaViS thinks it is spam.
AMaViS lets SpamAssassin check the email for spam. SpamAssassin will be taught which emails are spam to increase its detection accuracy.
AMaViS also runs the email through ClamAV to see if it contains any viruses.
After these checks AMaViS returns the email to the Postfix process but on TCP port 10025. Postfix is configured to trust emails sent to this port so further content checks are skipped.
Postfix forwards the email to Dovecot. Dovecot can optionally apply per-user filters so that emails can be stored in certain email folders automatically if desired.
Dovecot writes the email to the hard disk in maildir format.
The user’s email client can now fetch the new emails from Dovecot using the POP3 or IMAP protocol.