You can also leave sendmail running on your live server and specify that ALL mail gets relayed to the new server. (Maybe you described this already?) Coming from a hosting background with over 2 million messages per day, moving DNS lookups off to another box is a huge benefit.

You may also consider running a DNS service on that box, and have sendmail use the localhost to do lookups. The DNS service will be able to cache entries based on the TTL, and prevent over-the-internet lookups for domains that you have already delivered to.