Outlook 2016: Keeps asking for password

Lately we received a high amount of calls regarding issues with the repeatedly appearance of the password prompt of Outlook 2016. Our customers verified the correctness of their password by using OWA before, so this couldn’t be the root cause. Apparantly some clients were still able to send and receive mails, too.

As we actually did some infrastructural changes recently, we tried to find the cause of error on our Exchange-Infrastructure first. We’ve also enrolled the latest cumulative update, but no matter what we tried to do, the problem persisted.

What we could find out, was, that the problem came with Build 1711 of Outlook 2016. A downgrade to Build 1710 allowed us to get rid of that password prompt. So we’ve asked our customers to downgrade Outlook until we were able to find out the actual reason of this behaviour. Well, this didn’t work out well, most of the clients were updated over night to Build 1711 again…

This morning we found an reddit-post, that helped us a lot: https://www.reddit.com/r/sysadmin/comments/7pk33j/outlook_2016_password_prompt/

Apparently Microsoft patched in some changes for Office 365. The following registy-record:

[HKEY_CURRENT_USER\Software\Microsoft\Office\X.0\Outlook\AutoDiscover]
"ExcludeExplicitO365Endpoint"=dword:00000001

prohibits requests to Office 365-Endpoints during the Autodiscover-Procedure. It seems like Outlook tries to authenticate itself on those servers, which – logically – doesn’t work as we have our own infrastructure.

We’ve also created a ZIP-Archive with registry-files for Outlook 2010 – 2016. So if you don’t want to add this entry manually, you can download and unzip the package from http://c4y.eu/passfix. Then you can run the file belonging to your Outlook version.