Hi Russ. I have completed dozens of migrations of SmarterMail to new servers and when I do I always move over all of the .xml and .dat files from C:\Program Files (x86)\SmarterTools\SmarterMail\Service. That will ensure that you have everything in the configuration on the old server moved on to the new server.
Also, I'm not sure if there is a way to get the old server to relay all of the mail to the new server during the migration, but what I normally do it set up a 2nd free SmarterMail installation on a different server and use it as a Store and Forward. This will ensure all mail sent to your primary server during the downtime will be redirected to the 2nd free server and that server will hang on to it until your primary server becomes available after the migration. Once your new primary server is up and running, the Store and Forward server will send the mail to the primary server for delivery.
Mail's Best Friend
Authorized Reseller of SmarterTools Products
Authorized Reseller of Message Sniffer