Hi Andrew,
The quickest way for a server admin to move a lot of messages to another SmarterMail server would be:
1. Create a new folder in the web interface on the destination account.
2. Copy all the grp files in the source account's folder to that new folder using Windows Explorer.
3. Then delete mailbox.cfg in the destination account.
4. Login to the destination server's Web interface as the system administrator, go to "Manage -> Restore" and rebuild that account's destination folder.
If a domain admin or end user need to copy the files, they would want to using an email client and add both accounts via IMAP and then drag and drop the messages from one account to the other.
I hope this helps!