Incremental backup of mails

Hi
I am looking for a solution to do incremental backup of mailbox files in my organization. Earlier we were using rsnapshot to do the same . But due to the increasing file size and limited size of backup server , we are looking for any incremental backup tools. We are looking in to tools like rdiff-backup to do the same . Typical size of one mbox file is 1 GB and backup is done in internall server . Have any thoughts on this, . We are using dovecot for mails . Another alternative is to use mail directories instead of mbox files to avoid redundant data being backed up . Can you please share your thoughts / experience on this .