I am just about to start a Exchange On-Prem to Online migration, with setting up a hybrid connection. The problem occurs when I add the custom domain to 365, then verify it with TXT record. The users Outlook receive a 365 login prompt. This is Outlook 365 version. The Outlook Direct Connection seems to bypass On-Premise Exchange and looks to 365 for the domain. The users do not even have this custom domain added into their 365 accounts yet. I need a backend solution to stop Outlook from doing this. Something I can easily remove when migration starts/completes.