I have an existing Exchange 2003 server (working fine). I am currently transitioning to Exchange 2010. 2k3 will remain in place while I move mailboxes to 2k10.
When setting up the new server, I have set the external URL for 2k10 to mymail.domain.com. Currently users connect to the 2k3 instance at mail.domain.com. I have noticed further along in the migration that I (think) I can set it up so they could have the same external URL and I did not need to create a different external URL for 2k10.
Was this necessary? Can I keep the same external URL for both servers (they will be both running together for some time). I would prefer to keep it one URL ("mail.")