Just realized my server was set up according to:
The Perfect Server – Debian Lenny (Debian 5.0) With MyDNS & Courier [ISPConfig 3]
but it seems for squeeze only this one is available:
The Perfect Server – Debian Squeeze (Debian 6.0) With BIND & Courier [ISPConfig 3]
what do you suggest? Should I change my set up according to the BIND one replacing mydns?