Today all our SBS servers running Avast BP+ lost their Exchange Transport service with this error:
The transport process failed during message processing with the following call stack: System.NullReferenceException: Object reference not set to an instance of an object.
at Avast.ExchangeAntispam.HubAntispamAgent.SubmittedMessageHandler
Messages end up in PoisonQueue; Avast console is saying that it’s waiting on Exchange.
Some servers got cured by a restart, some had to have Exchange shield removed via reinstall.