what I do in a case like this is (and I know it may be a little archaic...but it works...):
1. stage new server completely - call it mail2.domain.com
2. on live server and new staged server bind a different LAN IP scheme to secondary NIC's
3. copy over all data from old to new
4. take main NIC of old server offline on pre-planned time/notice to customers. (We have our own front-door Spam/Virus filter server farm in front of the mail servers so all mail will still be received inbound into our systems, and queued up on these servers.)
5. perform a followup XCOPY to get any changes in data since the main data copy
6. rename server as mail.domain.com; IP the main NIC on server with IP of old server; double check all SmarterMail IP bindings
7. start all services; confirm everything once again
8. plug in main NIC of new server to network switch and all queued email on front end filter servers comes down
9. pour myself a drink and watch a movie