For an explanation of the symbols used, click on the icon in the upper left-hand corner of this page. Mail is posted locally. The Postfix sendmail program deposits the message into the world-writable maildrop directory, where the message is picked up by the pickup daemon. This daemon does some sanity checks, in order to protect the rest of the Postfix system. In order to avoid accidents, the directory permissions on the maildrop directory must be such that a user cannot delete someone elses mail.
Mail comes in via the network.
The Postfix SMTP server receives the message and does some sanity checks, in order to protect the rest of the Postfix system. The SMTP server can be configured to implement UCE controls on the basis of local or network-based black lists, DNS lookups, and other client request information. Mail is generated internally by the Postfix system itself, in order to return undeliverable mail to the sender. The bounce or defer daemon brings the bad news. This is indicated with the unlabeled arrow.