You will notice that there is a problem with mailflow and on the Queue Viewer in your Exchange setup you will notice that the mailflow is stuck for a good number of messages. You will also notice that the error message is 430 4.2.0 STOREDRV; mailbox logon failure.

You can also see the event viewer showing the Event ID 1009 in the Application log: The Microsoft Exchange Mail Submission service is currently unable to contact any Hub Transport servers in the local Active Directory site. The servers may be too busy to accept new connections at this time.

This can be because that the Microsoft Exchange Transport Service and Microsoft Exchange Transport Log services are running with a user and not the Network Service or Local System.

To fix this, open the Services.msc and set both services to Log On As Network Service or Local System. Restart both services and mailflow should get moving.