There was a interesting, rare, however stupid the situation is.
Is Windows SBS 2003 + Exchange. Crashes once in a hundred years, but it happens.
He is faced with postfix, sticking out. It seems, no crashes ever.
When Postfix accepts the email, looks at the available in AD email address, and if there is any forwards in exchange. If not bounces.
SBS fell, laying whole weekend. All weekend Postfix fought off all letters for checking the addressee could not.
Want in case of failure of SBS Postfix started to take all the letters if a relay is not available. Ie, putting off the procedure of validation and shipment, folding all the letters to better days.