A disclaimer rule that skips a message, an attachment that should have been stripped but was not, a sender who insists their email never arrived, complaints like these usually land on your desk hours after the message already left the server. Reproducing the conditions in a test tool only gets you a guess at what happened, not a record of it.

Policy rules can block, tag and rewrite messages on their own, but sometimes a match needs to trigger something outside the mail flow entirely - a chat notification, a ticket in another system, or a virus scan tool with no native Postfix integration. Building that hook yourself usually means writing and maintaining a custom milter just for one integration.

A rule that looks correct in the editor can still behave differently once mail starts flowing through it. Maybe the condition is narrower than you assumed, maybe another rule in the same group gets there first. Discovering that from a user complaint is the expensive way to find out.