Memorise

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

& $env:ExchangeInstallPath\Scripts\Disable-Antimalwarescanning.ps1

Note

To re-enable malware filtering, use Enable-Antimalwarescanning.ps1 instead of Disable-Antimalwarescanning.ps1.

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"

https://docs.microsoft.com/en-us/exchange/disable-or-bypass-anti-malware-scanning-exchange-2013-help

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Categorised as: Exchange, Microsoft