If all your mail is stored on the server (which it sounds like it is), then I think rsync is the easiest way to go. You could set up a cron job to run every hour or so and sync the directories. If bandwidth is an issue, you could instead write a script that tars and compresses your mail directory then copies it to the remote server and untar it.
Oh.. and don't forget to run rsync with "-e ssh"
Hope that helps