Outlook 2016 Connects First Time, Prompts for Password Every Other Time

A customer of mine recently deployed Outlook 2016.  The customer had an issue where they could create a new Outlook 2016 Profile, but after the Outlook Profile was created and Outlook was closed, the second time they open Outlook they would be prompted for password in an infinite loop.

Disabling Cached Exchange Mode on the Outlook Profile would fix the issue but we needed Cached Exchange mode to be enabled!

The protocol we were using is MAPI over HTTPs.

Outlook 2010 and 2013 connected fine with MAPI over HTTPs, only Outlook 2016 had this issue.

The issue turned out that the customer had disabled oAUTH on the MAPI over HTTPS virtual directory.

Below is the authentication settings setup on the customers Exchange 2016 server:

This is what the default authentication settings look like:

To fix the issue we needed to set the settings back to default with the following command:

Get-MapiVirtualDirectory | Set-MapiVirtualDireectory -IISAuthenticationMethods NTLM,OAuth,Negotiate -InternalAuthenticationMethods NTLM,OAuth,Negotiate -ExternalAuthenticationMethods NTLM,OAuth,Negotiate

Make sure you reset the correct IIS App Pool or do an iisreset so the change takes effect immediately.