Outlook bypasses AutoDiscover and connects directly to Office 365 mailbox
Starting in Outlook 2016 version 16.0.6741.2017, Microsoft has enabled a new feature called Direct Connect to Office 365. What this feature does is connect Outlook directly to Office 365 if Autodiscover is not working. This is great feature but a network hiccup may cause your mailbox to connect to Office 365 rather than your on premise Exchange, even with a valid autodiscover record.
For Direct Connect to take effect the user must have an active mailbox on Office 365 with a valid license. Typically Direct Connect would be a neat feature unles you are in the middle of a migration to 365, but a network hiccup could incorrectly connect you to 365 even though you haven't completed your migration yet.
We recommend disabling this option from the user computer registry:
You can stop Outlook from checking Office 365 for settings by setting a registry key.
HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\office\16.0\outlook\autodiscover DWORD: ExcludeExplicitO365Endpoint Value = 1
Then restart your computer and it should stop trying to connect to Office 365. When you are ready to flip the switch on your migration, remove this registry key.
or
\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover