Help migrating mailboxes from one Ensim dedicated server to another

I am upgrading to a new dedicated server. Both are running Ensim, but are running different versions. I am having trouble backing up the mailbox accounts and migrating them to the new server. All other files and databases are installed. I just have the mailboxes left. I have used Ensim's own backup and restore tool, but because of the size of the entire site, it fails. Root and SSH access is available on both machines. Please ask any questions you may have about this project before bidding. Thanks.