I would verify that their Exchange account is the only account within outlook. From a few different places it looks like it could be something stuck in Drafts too (although you said you checked there). Possibly removing the account and re-adding it in Outlook could be the quick fix to it (although they would lose their saved autocompletion which a lot of people complain when they lose it).
Im running 2016 on probably 25 Mac's and haven't seen this issue yet, but I have had weird issues with Outlook in general that aren't worth the time messing with and just adding the account back again will fix saving all of our times.
Office 2016 For Mac Outlook Could Not Syn Record To Exchange Server Instance
This problem occurs because one or more default folder in Outlook for Mac 2011 were moved or deleted. These folders include Conversation History, RSS Feeds, Sync Issues, Local Failures, and Server Failures. You cannot move or delete these default folders in Outlook for Mac. After that, whenever a user tries to synchronize Exchange Server with Mac Outlook 2011 application, the user certainly receives the below-described error: Error: Could Not Synchronize Record Outlook 2011 Mac. After that, whenever a user tries to synchronize Exchange Server with Mac Outlook 2011 application, the user certainly receives the below-described error: Error: Could Not Synchronize Record Outlook 2011 Mac. The error section says: Could not synchronize record: Call to Exchange server [email protected] Details section says: An internal server error occurred. I'm running Mavericks and my Mac Mail will autocomplete addresses that are in the GAL but I would like to see the entire list so I can see distribution lists and the corresponding members. There are solutions to sync the GAL to an Outlook contacts folder in the each Exchange mailbox. This is usually done for the benefit of iPhone users with an.