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Postfix Documentation
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What mail is subjected to header/body checks

Postfix header/body checks are implemented by the cleanup(8) server before it injects mail into the incoming queue. The diagram below zooms in on the cleanup(8) server, and shows that this server handles mail from many different sources. In order to keep the diagram readable, the sources of postmaster notifications are not shown, because they can be produced by many Postfix daemon processes.

\ |
cleanup(8) -> incoming
/ ^

For efficiency reasons, only mail that enters from outside of Postfix is inspected with header/body checks. It would be inefficient to filter already filtered mail again, and it would be undesirable to block postmaster notifications. The table below summarizes what mail is and is not subject to header/body checks.

Message type Source Header/body checks?
Undeliverable mail bounce(8) No
Network mail smtpd(8) Configurable
Network mail qmqpd(8) Configurable
Local submission pickup(8) Configurable
Local forwarding local(8) No
Postmaster notice many No

How does Postfix decide what mail needs to be filtered? It would be clumsy to make the decision in the cleanup(8) server, as this program receives mail from so many different sources. Instead, header/body checks are requested by the source. Examples of how to turn off header/body checks for mail received with smtpd(8), qmqpd(8) or pickup(8) are given below under "Configuring header/body checks for mail from outside users only" and " Configuring header/body checks for mail to some domains only".

Postfix Documentation
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