How does a non-domain joined Outlook client determine Internal MAPI vs External HTTPS access?

This is for Exchange 2010 fyi.

Does it try to resolve the MAPI address? Does it try to access Active Directory?

I tried removing the MAPI address (CAS Array) from DNS but it's still trying that address for internal access... what else could it be looking for? We do not have split DNS so we are trying to prevent external clients from waiting waiting waiting, and then timing out.