Users with Exchange 2007 mailboxes in their profile should not upgrade to Office 2016 as they will be unable to connect to their Exchange mailboxes. When they try to connect they'll receive the following error message:
The resource that you are trying to use is located on an unsupported version of Microsoft Exchange.
Historically, Outlook is supported on three versions of Exchange server. In the case of Outlook 2016, those three versions are Exchange 2016, 2013 and 2010. All Exchange servers in the network need to be on Exchange 2010 or newer.
The short term fix: you’ll need to re-install Office 2013 if you want to connect using Exchange services and access everything in your mailbox. If you only need to access email, use IMAP.
The long term fix: Exchange 2007 needs to be upgraded to Exchange 2013 or Exchange 2016 if you want to continue to use Outlook 2016.
If you need to view the calendar, set the account up in the Windows Mail app and only sync the calendar.
If you need to connect to Exchange 2007, you can disable the automatic upgrade by setting the following registry key (create the keys if they are missing):
You'll need to close and restart Outlook for values under the policy key to apply.
Do It For Me
If you don't want to edit the registry yourself, you can download this ready-to-use registry file. Double click to run. enableautomaticupgrade
Users who were auto-upgraded to Office 2016 from Office 365 Personal, Home, or University who need to revert back to Outlook 2013 will need to contact Microsoft support by selecting Contact Answer Desk on the Microsoft support page.
Business subscribers should have time to block the upgrade using the registry key, if not, see Obtaining older versions of Office for the steps necessary to revert to Office 2013.
Published August 2, 2015. Last updated on September 29, 2015.