Outlook 365 prompting for login credentials

We have an SBS 2011 server which is working well. We have just upgraded one of our office Windows 10 laptops to MS Office 365. The problem we have is that when Outlook opens it it pulls down all the new emails etc and is connected to Exchange. As soon as it finishes this updating process a windows credentials box appears and asks you to put in the password. In the username it has the correct email address but shouldn't this be the users username not email. Either way if you put in the password it accept this and connects again and then after a small random time period it does the same thing. Its like it forgets the credentials.

We have tried deleting the credentials in credential manager but same thing  

Any help would be appreciated?
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FOXConnect With a Mentor Active Directory/Exchange EngineerCommented:
It should be using the UserPrincipalName and the password.  If the UserPrincipalName is the same as the user's email address you are good to go.  Make sure you put a check in "Remember Credential"

You can also try this-
1.  Make sure credential manager is cleared out again
2.  Close outlook and reopen it in safe mode.   Start> in the run box type       outlook /safe    press Enter
3.  When prompted put in the user's UserPrincipalName, password and put a check in "Remember Credentials"
Omar SoudaniConnect With a Mentor System Support EngineerCommented:
Try removeing cached mode in Outlook, and see if issue happens.

In a similar case some time ago, I put Outlook 2016 in online mode (not cached) and prompt disappeared. Put it back to cached mode and all was good.

Saurabh YadavConnect With a Mentor MCPCommented:
Please try to uninstall the office 2016 then try to install the click to run version downloaded from the portal and also try to clean all the registry entries from the laptop and clean the credential managers. Please follow the below mentioned URL to remove the office 2016 setup manually.


Hope this should work !
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