Backing up the users' mail folders

[Buday, Tomas]

Dear all,

  I'm successfuly running Qmail-LDAP on two systems in a cluster 
environment and wondering of the best way to backup the mail folders
online so that nobody notices anything. I've got over 10K users.

  Can you please reply with your experience and the possibilities. 
Are there any open-source solutions? I know Veritas Netbackup and
HP Omniback would do.

  Regards,

Tomas Buday