A feature in Outlook 2016 will make sure that Outlook connects to Office 365 if the e-mail address and mailbox exists in Exchange Online. This can cause issues during an Office 365 migration when you still are using your current on-prem mailbox while you also require a Exchange Online mailbox in order to copy the mailbox data to Office 365. This can be fixed by adding/altering a registry value.
In order to resolve this issue you need to set the registry value below to 1. If the registry value doesn’t already exist, create it. The registry value type is DWORD.
You can also paste the following text into notepad, save it as a .reg-file and run it on the affected machines to update the registry and resolve the issue:
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover] "ExcludeExplicitO365Endpoint"=dword:00000001
After setting this you will need to recreate the Outlook-profile for the affected user. Now Outlook should use the autodiscover records to determine what server to connect to.
When planning a migration it would be wise to deploy this registry setting through group policy if your machines are domain joined.