One of our client called saying that all thier Email went to Draft Folder in OWA and cannot Send & Receive Email via Exchange 2016 Server.
We do not see any error message in Event Viewer until we perform SMTP Telnet test on the Exchange Server
4.7.0 Temporary server error. Please try again later. PRX2
The root cause of this issue was they configure the alternative DNS Server to point to 18.104.22.168 (Google DNS Server) in Exchange 2016 Server.
Once we removed the DNS Google DNS Server, and only pointing to our internal AD/DNS Server, Exchange 2016 Server is back to normal and users can send & receive Email now
It is NOT required to point alternative DNS Server to External DNS if you only have a single AD Domain Controller as AD/DNS will help to resolve FQDN automatically