In on-prem Exchange, some limits are set for the number of sessions a user can open. If the user has many connections to different calendars and mailboxes, these limits can be reached. On the Exchange server, in the Application Event Logs, you can see entries like this: EventID 9646 - Mapi session [..] with client type WebServices exceeded the maximum of 16 objects of type Session. or EventID 9646 - Mapi session [..] exceeded the maximum of 32 objects of type Session. These 2 errors cannot be Read more [...]