Last Error: Message deferred by categorizer agent.
Exchange 2013 mail flow issue. It turned out to be the Malware agent causing the issue
Once it is enabled and the Exchange transport service is restarted, all emails are stuck in the submission queue with “Last Error: Message deferred by categorizer agent.”
Check the mail queue
Get-Queue -Identity submission
Get-Transport Agent will list all the transport agents, you can disable one at a time to isolate problematic agents. in my case it was the Malware Agent.
Use the Shell to disable malware filtering on a specific Exchange server
To disable malware filtering, run the following command: PowerShellCopy
To re-enable malware filtering, use
Enable-Antimalwarescanning.ps1 instead of
How do you know this step worked?
To verify that malware filtering is disabled, run the following command and confirm that it returns a value of False: PowerShellCopy
Get-TransportAgent "Malware Agent"