Outlook 2016 - Autodiscover defaults to Office 365
I recently came across an issue where Autodiscover failed in Outlook and it defaults to Office 365 Autodiscover (something like this - https://autodiscover.outlook.com/autodiscover/autodiscover.xml). As the mailbox is hosted on premises, Outlook keeps prompting for credentials.
You can check this by Holding Ctrl key and Right Click on Outlook icon. Then Click Test E-mail AutoConfiguration, as shown in below screenshot.
Type in the email address, password, untick Use Guessmart and Secure Guessmart Authentication. Then Click Test. Check the Log tab to see if it defaults to Office 365 Autodiscover - if it does you should see it in very first line.
As mentioned here, this affects Outlook 2016 version 16.0.6741.2017 and later versions.
How can this be fixed?
It can be fixed by adding the below registry key.
Open Registry Edit on the PC (Search or in the Run box type regedit.exe)
Go to the path: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
Create ExcludeExplicitO365Endpoint DWORD or if it already exists set its value to 1
Restart the PC
Test E-mail AutoConfiguration again. Provided Autodiscover is configured correctly for the domain it should all work now.
I hope you found this useful.