Outlook defaulting to outlook.office365.com instead of On-Premise Exchange causing extreme slowness

[JPForth]

Hi Community

  My company has an odd issue going one since the release of SAEC Version 2102 in July.  Any net new install has Outlook sending autodiscover traffic to outlook.office365.com instead of our on-premise exchange server. 

Our exchange infrastructure is all on premise and we block access right now to outlook.office365.com.  This is causing extreme slowdowns for affected users in Outlook.  The mail profile takes 5-10 minutes to load and moving around the mailbox takes 2-3 minutes when clicking between emails and other areas like calendar, and especially shared mailboxes. 

I'm personally not able to reproduce this issue on my own mailbox.  I've got a premier case open, but the engineer seems to be focusing on the outlook.office365.com portion rather than the over all issue.  We've deleted OSTs and rebuilt email profiles to no solution.  Rolling the users back to Office 2016 ProPlus MSI resolves the issue.  If we reinstall M365 Apps the issue presents itself again. 

We use Cached Exchange mode, and use the defaults for cache sync.  We've tried changing the amount of data to cache as well. This happens with regular user inboxes with our without shared mailboxes attached. 

Has anyone encountered this or have any tips?  Could it have been an issue with initial 2102 download for SAEC? 

Thanks in advance!

[#2 · Accepted solution]

See if this helps in any way https://docs.microsoft.com/en-us/outlook/troubleshoot/profiles-and-accounts/unexpected-autodiscover-...

(I know adding the ExcludeExplicitO365Endpoint has worked for similar issues).

[#3]

Thanks. That seems to be the issue, the impacted devices are ignoring that registry setting. We have it set in policy.