When creating custom filter rules, one of the available actions is to "deep filter" the message. What exactly does it mean to deep filter a message?
The deep filter action temporarily quarantines messages that match the custom filter rule. By default, quarantined messages will be held for about two hours and then re-scanned. While the message is being held, it is displayed in the quarantine viewer and can be manually released or marked as spam. The second scan uses the same process as the original scan, except the time delay provides additional time for new spam campaigns to hit our spam traps resulting in new rules being created. At the end of the two hours, if the message still does not match any of the spam blocking rules in our global database, it is then released.