Users connecting to Exchange 2007 mailboxes should not upgrade to Office 2016 as you cannot connect Outlook 2016 to Exchange 2007 using Exchange services. If you only need email, you can use IMAP, provided IMAP support is enabled on the server.
Users who upgrade may see the following error message (assuming the account was not removed from the profile on first run):
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 (or your previous version of Office) 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 migrated to Exchange 2013 or Exchange 2016 (or Office 365) if you want to continue to use Outlook 2016.
Tip: If you need to view a calendar stored in an Exchange 2007 mailbox, set the account up in the Windows Mail app and configure it to sync only the calendar. (The calendar app uses Exchange Active Sync.)
Businesses using Group Policy can block the automatic updates using the updated Office 2013 group policy template. Look for the option to block updates under Computer Configuration\Administrative Templates\Microsoft Office 2013 (Machine)\Updates.
Double click on Enable Automatic Upgrade and choose Disabled, then click Apply and close the dialog. If Outlook is open, close and restart it.
Anyone can block the updates 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.
Users who were auto-upgraded to Office 2016 from Office 365 Personal, Home, or University and 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 plenty of time to block the upgrade using the registry key, if not, see Obtaining older versions of Office for the steps necessary to revert to a previous version of Office.