After your organization is upgraded to the December 2016 update for Dynamics 365, existing mailbox records may quit synchronizing and the following alert is logged:
"Mailbox <Mailbox Name> didn't synchronize because the user has an outdated client ID. We have reset the information for you.
Email Server Error Code: Crm:80044113."
If you click Details, the details section includes text like the following:
"subscriptionclients with clientid <GUID> doesn't exist"