When you are migrating to a hosted Exchange service but you want to keep your existing Exchange server for a little while you deal with the problems you run into with Outlook on domain computers. Outlook automatically will connect to the Exchange server listed in AD rather than the one you specified when you setup the Outlook profile (at least that was the case when I was dealing with this). So even if you use an external address in the server config, after you open Outlook it’ll automatically revert you to the Exchange server listed in AD.
So what you need to do is remove the AutoDiscover and SCP entries from Active Directory…
- Delete the Autodiscover virtual directory in IIS manager on the Exchange server(s). Path: IIS manager > Default Web Site > Autodiscover.
- Delete internal DNS record related to Autodiscover: autodiscover.domain.com and mail.domain.com.
- Delete the SCP record using ADSI edit: Login to a server running AD and open ADSI edit. Expand down until you get to CN=Autodiscover and then delete the object that is of type serviceConnectionPoint. You may have to delete it on multiple servers depending on the architecture of your Active Directory.
ADSI path: CN=Autodiscover,CN=Protocols,CN=,CN=Servers,CN=Exchange Administrative Group,CN=Administrative Groups,CN=First Organization,CN=Microsoft Exchange,CN=Services