We are currently working on phasing our old (aka ancient) Exchange 2003 server out of our organization after upgrading to Exchange 2010. After attempting to artificially remove (either completely shutting it down, or changing the routing connector to a higher digit priority) Exchange 2003 to make sure Exchange 2010 is fully functional by itself we can't send email outward. We can receive email from outside the org, and we can receive and send from inside. So essentially everything works without the Exchange 2003 server functional EXCEPT sending out of the organization.
While watching the queue viewer while testing I can see the email get stuck in DNS Connector Delivery Queue and eventually give an "unable to connect" error. I've tried allowing port 53 and port 25 both directions on both the local and network firewall, which didn't work. I've also ran nslookup on the external DNS settings and it returns IPs for domains so it definitely can connect to the external DNS server.
Worth noting is that when I do this test I do still have the inter-exchange connector between Exchange 2010 and Exchange 2003 up, but I have it's priority on Exchange 2010 set to a higher digit value than the internet send connector. In normal operation the Exch2010-Exch2003 connector would be the only send connector enabled.
Our business is 24/7 and is required to not have any downtime on email. We can bring it down but not for long enough to lose emails to TTL. So essentially no longer than an hour if we can help it. So any awkward things like uninstalling and going through the removal process of 2003 to test isn't going to cut it.