Are you able to telnet to the exchange server and see if you have incoming/outgoing flow, to/from the AR Server? If not, you might need to check with your Exchange Admins on what has been changed on their side. Also, after restarting the Email Engine, can you monitor any other errors in the email logs.
sorry, i was wrong. outgoing mail is working. only incoming is not.
The error you reported looks like it contains a GUID - the value AGGAA5V0G7GPAANEZTWHAC0BWTHOEV. I wonder if there's a problem parsing an inbound message? Check the messages form and your mailserver inbox for a message with this string, remove it and restart the email engine.
We have same issue in 9103 prod. The emails are going out. We see java is using almost 70% memory usage which could be problem.
We will try by increasing java heap size and check.
Do you see same problem?