Exchange 2016:- Event ID 9646 – AirSync exceeded the maximum of 250 objects of type Message.
The above error logged in Exchange 2016 is a strange one. You may see the following error below logging in the event log quite often for 1 or 2 users:
While you can go and increase the registry key below but this won’t resolve your issue as you just making the value larger in this key and it will eventually consume all the mapi connections:
Look for the Key: MaxObjsPerMapiSession and increase it’s value.
What I did find is that if you use an older device that has an older ActiveSync version, you may want to upgrade the OS of the device and then see if the error goes away for that user/s.
Hope it helps.