Backup Exchange Server 2000/Win2k Server

I have a main office having Windows 2000 server active directory domain running exchange 2000 server on it. Goal is to have a backup exchange server in one of the 5 branch offices, if the main office completely is cut off from the branch offices. All offices are connected via the Internet by Sonicwall VPN. I want the backup server to be functional with all mailboxes when main office is cut off. No need to sync always. I’m looking for best backup/restore solution.