You are setting up a non-Office365 mailbox or an on-premise Exchange mailbox, but it is detecting your Office365 account even though the Autodiscover is not pointed to Office365.
Microsoft launched the “Direct Connect to Office365 Feature”(Outlook 2016 version 16.0.6741.2017 and higher) that forces your Outlook 2016 to connect to Office365 when:
- You have users that are not yet active to Office365 has a mailbox provisioned to it due to Exchange license assigned
- Autodiscover is not working on the source server.
- Connection of the computer and the source server is interrupted or blocked by firewall/antivirus
This feature is great since it would automatically detect your Office365 account; however, it can also be a huge problem when you’re in the process of migrating to Office365.
When this happens, email continues to work; however, the user is not going to receive any emails to Outlook since it’s connected to Office365; the email will be delivered to Hosted Exchange.
The solution is to apply registry keys, details below
- Download the archive at the following LINK (ExcludeExplicitO365Endpoint.zip)
- Open the archive with a special program (e.g. WinRAR, 7zip or Windows Explorer)
- Open the DisableO365.reg file inside the archive (e.g. by double clicking it)
- If prompted, allow "Registry Editor" to make changes on the device (enter the administrator password if necessary)
- Click on the “Yes” button when the following dialog box appears.
- Click on the OK button
- Restart Outlook
