We have a forum article discussed this issue in detail, please follow the instructions in the article to troubleshoot the issue: https://social.technet.microsoft.com/Forums/en-US/bcd2d9c2-1a1b-4446-bf32-69fee8cdf11b/why-does-outlook-keep-prompting-for-password?
Users connecting to Exchange 2007 mailboxes should not upgrade to Office 2016 as you cannot connect Outlook 2016 to Exchange 2007 using Exchange services.
In the case of Outlook 2016, those three versions are Exchange 2016, 20.
For instance, setting the value to 4 will trigger a full download when the incremental update is larger than one-fourth the size of the full OAB.
When the value is set to 0, Outlook will use 16 to prevent a “divide by 0” error.
If the key name doesn’t exist, Outlook uses a value of 8.
Prompt before for full download As the OAB can be quite large in some environments, it could be wise to be prompted when Outlook wants to download the full OAB.
In that case the whole OAB is being downloaded again.
In a later section, it is explained how to modify this.
This problem seems to have been around for a while yet is not acknowledged by Microsoft and none of the forums yet have solutions posted that work despite it being very well communicated.
As the push for O365 users to upgrade to Office 2016 is starting, this might become a big headache for many people.
(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.
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.