My company uses an SBS 2003 server and makes use of Exchange to host our own email. We also have a linux server hosting domains for some of our clients. In order for us to send to those clients, we had internal DNS set up to shadow the client domains to provide "correct" MX records inside our network. For example, public DNS for a domain abc.com might point to 22.214.171.124, but internally we have MX records set up to route mail for abc.com to 172.16.0.4, which is the linux email server. This setup was entirely functional; this is just back story.
We've recently moved one of our client domains from our internal linux server to an external email provider. When we did that, we naturally deleted our internal shadow DNS records so our Exchange server would fetch correct (public) DNS records and route mail out to the new external host. This has NOT had any effect on Exchange though. Even after rebooting the Exchange server and completely flushing the DNS cache (nslookups on the Exchange machine itself correctly resolve to the new external address) Exchange still attempts to deliver messages for the domain to our internal server! Exchange correctly routes to all other internal and external domains when sending email.
Somehow Exchange is trying to deliver to a machine that by all accounts it has no business trying to use for just this one domain. Is there a DNS cache that Exchange uses internally? Is there a way to flush that internal cache? What else could I be missing?