Last Error: Message deferred by categorizer agent.

Good day.

I have Exchange 2016 CU8 installed. Two nodes setup with DAG. It works fine until I tried to enable the Malware Agent. Once it is enabled and Exchange transport service is restarted, all emails are stucked in submission queue with "Last Error: Message deferred by categorizer agent.".

I tried to use Set-MalwareFilteringServer to bypass the scanning but mail flow still not working.

However as soon as I disabled Malware agent, the mailflow is back working right away. 

Any suggestion? Why malware agent doesn't work well?

Thanks!