Following the migration of Mailboxes from Exchange 2010 to 2013/2016 users will receive the expected:
But then they may not be able to open Outlook:
"Cannot start Microsoft outlook. Cannot open the Outlook window. The set of folders cannot be opened. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance."
This is a known issue https://support.microsoft.com/en-us/help/3097392/outlook-logon-fails-after-mailbox-moves-from-exchange-2010-to-exchange and the resolution is to restart the Autodiscover Application Pool on the Exchange 2016 server/s (Restart-WebAppPool MSExchangeAutodiscoverAppPool).
As there is no permenant solution offered, an option would be to configure the Application Pool to recycle after an elapsed time (https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/cc733120(v=ws.10)?redirectedfrom=MSDN) for example 5 minutes and then worse case a user would have to wait up to 5 minutes before they would be able to reconnect.