UPDATE Microsoft has published the solution to fix the issue; please check the source link at the end of this post
This is not a good start for Exchange administrators. If you are using Exchange on-premises, your emails may be stuck in the queue with the error
Message deferred by categorizer agent
This is due to the antimalware engine which has trouble to validate the date after the start of the new year.
While Microsoft is working on a fix, the only option available is to disable the antimalware engine using the PowerShell script and restart the Exchange Transport Service