Reviewing the Brightmail Engine logs you find instances of the following warning level message:
2022-08-04T12:39:30-04:00 (WARNING:10726.108807936):  Rulesystem-reinit request received with 4 rulesystems already in memory; request deferred.
Component : Brightmail Engine
This means that there are some long running message scanning transactions in progress and the Brightmail Engine cannot load a new rule update as all available rule set "slots" are in use but running scanning transactions.
Usually this doens't represent a significant issue and resolves itself through normal product operation. If, however, these messages continue to be logged for an extended period of time (an hour or more) it can mean that loading of new anti-spam rules is being delayed and the removal of a potentially non-performant rule is not happening. In this case the Brightmail Engine should be restarted to force a reload of the latest anti-spam rules from disk.
To restart the Brightmail Engine
The Brightmail Engine can also be restarted from the SMG admin command line by running
service bmserver restart