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
& $env:ExchangeInstallPath\Scripts\Disable-AntimalwareScanning.ps1
Restart-Service MSExchangeTransport
Thank you for your post, I’ve been searching this very issue for the past 6 hours.
You’re welcome
Microsoft has published the solution to fix the problem while keeping the antimalware engine enabled; please check the source link at the end of the post