Outlook Direct Connect to Office 365
I received several panicked calls from one of my clients today - while they are away at a race meet, one by one their Outlook mailbox connections are dropping out and becoming stuck in a never ending loop of prompting for credentials.
Starting in Outlook 2016 version 16.0.6741.2017, Microsoft has included a new feature called Direct Connect to Office 365. It’s designed to force Outlook to connect to Office 365 if the user has a mailbox, in order to make it easier for end users to configure Outlook profiles. While it certainly is a neat feature, if you’re in the middle of a migration from on-premise Exchange to Office 365 (like they are!) it can cause a great deal of confusion and wasted time trying to troubleshoot the issues.
Fortunately, the fix is quite straight forward but not very well documented. There are two options, a local registry key setting or disabling MAPI on the mailbox in Exchange Online (we opted for the latter to perform the change en masse).
HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\office\16.0\outlook\autodiscover DWORD: ExcludeExplicitO365Endpoint Value = 1
Disable MAPI using EXOP
Set-CASMailbox -Identity $username -MAPIEnabled $false