I have a client that has an on-premises Exchange server. The plan was to replace this with a membership to Office 365 as it was costing alot of money for maintenance etc.
We did not do a conventional Migration, basically I recreated all the users manually in the Exchange 365 admin portal (20 users) and then changed over DNS and setup new profiles in Outlook 2007. PST files were recovered from the old server and used on the client's workstations as archives.
The issue I have now is that Outlook will connect to the new 365 accounts but will consistantly disconnect and prompt for credentials for the old server. Autodiscover has been tested and there do not seem to be any GPOs set that would affect this.