This is only a short blog post to document a solution for a very annoying problem. After the automatic update of my Outlook to the latest Office 365 build (version 1809), it has started to prompting for credentials. I’m using Outlook to access a Microsoft Exchange 2016 server (on-premises), without any hybrid configuration. A pretty simple and plain Exchange 2016 on-prem deployment.
I knew, that it has to be related to Office 365, because the Outlook 2016 on my PC at the office was not affected. Only the two Office 365 deployments on my ThinkPad T480s and ThinkPad X250.
To make this long story short: ExcludeExplicitO365Endpoint is the key! You have to add a DWORD under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover.
Value = 1
Restart your computer and the annoying credentials prompts are gone.