I read this thread
https://portal.smartertools.com/community/a2830/i-recently-deleted-a-domain-but-now-it-needs-to-be-restored-from-backup_-how-do-i-add-it-back.aspx
Why not making it possible in the webinterface like restoring a user.
Just restore the domain from backup to the right location (absolute path on server)
Choose restore domain, give in path (like creating new domain) and the domain gets restored.
TMaybe that's possible without restarting smartermail service like creating a new domain? But that I am not sure because I'm no developer.