Outlook 2016 / 2019 Prompts for Password after initial setup and connection to On-Premise Exchange 2016 Server

I rebuilt my Windows 10 VM Image for XenDesktop and was testing it. I ran Outlook 2019, which configured successfully, opened my mailbox, etc. I logged off, did some tweaking to the image and logged in again. 

When I launched Outlook it was prompting me for a password. So I typed it in, but it would not log me in. Changed the logon from the UPN displayed to the DOMAIN\USER format, which still didn't. 

I decided to check the Security and IIS Logs of the Exchange Server and Domain Controllers for logon failures, which turned up nothing. But how can that be? I failed to logon on somewhere.

I looked at WireShark captured packets and noticed I was going to a Microsoft address which looked like it might be for Exchange Online Services or O365. I do use Exchange Online Services for my SMTP relay, Virus Scanning, Spam control, etc. Maybe Outlook was trying to sign me in there?

Doing some searching with various keyword combos I came across this: https://support.microsoft.com/en-ca/help/3211279/outlook-2016-implementation-of-autodiscover

Particularity, this:

"Step 4: Check for O365 as priority

Outlook uses a set of heuristics to determine whether the provided user account comes from Office 365. If Outlook determines confidently that you are an O365 user, an attempt is made to retrieve the Autodiscover payload from the known O365 endpoints (typically https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml or https://autodiscover-s.partner.outlook.cn/autodiscover/autodiscover.xml). If this step does not retrieve a payload, Outlook moves to Step 5."

Well isn't this lovely and strange. I stopped using my account and tested another account and it worked fine, every time. Why? Because they are not Exchange Online Administrators!

In the end, I had to set the following Registry Items to 1:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
Value Name: EnableOffice365ConfigService
Value Type: DWORD
Value Data: 1

HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\Office\16.0\Outlook\AutoDiscover
Value Name: ExcludeExplicitO365Endpoint
Value Type: DWORD
Value Data: 1

Crazy thing is, this did not happen on my Laptop. This only happened when using the VM, which uses Citrix Profile Management, so maybe I am not saving something in the profile after the initial configuration. Who knows, moving on...