Outlook on Exchange On-Premise Credential Re-prompt

The Problem

On a Windows 10 Pro PC running Outlook 2016, connecting to an Exchange 2013 on-premise server - a user was frequently receiving prompts for their username and password in an Office 365-style Modern Authentication window.

Having confirmed that the user name and password were definitely correct, it was time to start Googling for a solution, since these sorts of issues rarely occur in isolation…

It took some digging, but I happened across a solution which seemed to fit.

The Explanation

With thanks to user “tedman” from Spiceworks…

In the newest version of Office 2016, it looks for Autodiscover records at 365 first, before trying to use SCP etc. Older versions try to use SCP and HTTP methods, but now 365 is the default.

This can be disabled with this reg key, which stops Outlook from trying to poll 365 autodiscover records first.

tedman

The Fix

The below registry key overrides this new behaviour in Outlook:

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover]
"ExcludeExplicitO365Endpoint"=dword:00000001

This key didn’t exist, so I added it, then the DWORD with the indicated value.

Then I closed and re-opened Outlook 2016, and the password re-prompt did not reoccur.

Just to be on the safe side, I then rebooted the PC and opened Outlook again - and the issue remained solved.

The PowerShell Way

If you’d prefer a quicker way to add the above key to the registry, see below:

New-Item –Path 'HKCU:\Software\Microsoft\Office\16.0\Outlook\' –Name AutoDiscover

$newPropertyParams = @{
  'Path'  = 'HKCU:\Software\Microsoft\Office\16.0\Outlook\AutoDiscover'
  'Name'  = 'ExcludeExplicitO365Endpoint'
  'Value' = '0x00000001'
  'Type'  = 'DWORD'
}
New-ItemProperty @newPropertyParams