This will be another short one. We got several feedback a while ago regarding a slow loading Outlook profile. It seems this started after an Office 2016 upgrade and occurred across users from different clients. These are situations where our scope of action is quite limited. We provide the server infrastructure and host the mailboxes. The Outlook clients have not been purchased through us and we have no real responsibility for the client side.
Exchange was not the culprit otherwise the problem would have affected all our clients. We assumed autodiscover respectively the order the affected clients connect as the cause, since we saw a similar behavior before > Outlook continuous password prompt
To keep it short we found the solution here :
In this YouTube video Grant Brown brings it straight to the point after he wiresharked the domains Outlook tries to connect to. ( Thank you ! )
The solution finally is to block Outlook from connecting to office365.com
Open the hosts file as admin and add the following entry:
This will prevent the client to connect to outlook.office365.com.
You can find the hosts file here :
A slow loading Outlook profile under these circumstances seems to occur when the mailbox is hosted elsewhere than on office 365 servers and an Outlook Click To Run client is used. We have suggested this fix to all affected Outlook users and it never broke anything.
Take in account to test this fix when you connect to O365 mailboxes and OnPremise from the same computer before you apply it.
Have nice day