Today, I came cross another interesting mail flow issue, where all mails stuck in Draft folders for all users when they are using OWA. You can imagine that mail flow was broken, that non of users can send any mails internally or externally.
Customer has troubleshot it for over 12 hours, and has gone as far as re-install operating system and Exchange 2013 server with /RecoverServer switch, but issue remains.
When I started looking at the issue, I went through series of basic transport troubleshooting steps for Exchange 2013 multirole server, such as checking all transport related services, possible back pressure issue, and state of all server components. Of course, there is nothing wrong with them.
Running out of ideas, I checked settings of send connector, just to make sure there is nothing out of ordinary. I see this in Send Connector properties,
There are not many reasons for any Exchange server to use External DNS server for lookups out there. For this environment, it certainly is not needed as well.
I unchecked the box, and restart transport service to speed up the process, but issue remans.
I then run get-TransportService | fl *dns*, to make sure that we don’t have any external DNS settings configured.
Ah ha! External DNS server setting is set. I run few tests with nslookup, the DNS server did not respond to any queries. So that’s probably the reason why that mails are not flowing.
To remove it, you have to run Set-TransportService -ExternalDNSAdapterEnabled $true -ExternalDNSServers $null.
After restarting the transport service, all mails in the Draft folder are gone. Mail flow is restored!