Outlook clients are connecting to Office 365 before cutover.
Starting in Outlook 2016 version 16.0.6741.2017, Microsoft has enabled a new feature called Direct Connect to Office 365. This feature connects Outlook directly to Office 365 if Autodiscover is not working.
If Autodiscover is not working on the source server or there is a network hiccup between a computer and the source email server then Direct Connect may cause Outlook to connect to Office 365 even with a valid Autodiscover record pointing to the source.
To prevent the profile from connecting to Office 365 there are two workarounds:
- Registry Edit
- CAS command in Office 365 to disable MAPI
To Edit the Registry:
- On each affected computer open the Registry Editor and navigate to this registry branch: HKEY_CURRENT_USER\SOFTWARE\Microsoft\office\16.0\outlook\autodiscover
- Set the value DWORD: ExcludeExplicitO365Endpoint Value=1
- Note: If the value does not exist, it will need to be added and set to 1
- Restart the computer.
To use CAS Command to Disable MAPI for a mailbox:
- Refer to this Microsoft article: Enable or disable MAPI for a mailbox
- Example: Set-CASMailbox -Identity "username" -MAPIEnabled $false
After permorming one of the workarounds you will need to create a new profile that connects to the source. A new profile is required because the msExchMailboxGuid of the source server is different from the msExchMailboxGuid in Office 365.
- At cutover you will need to remove the registry edit or use the enable CAS command to enable the Direct Connect feature.
- For more information on Microsoft Implementation of Outlook 2016 Autodiscover see this Microsoft article: Outlook 2016 Implementation of Autodiscover