> Thanks for the tip, but how do I configure postoffice
> to act as a backup mailserver for a domain?
1) Set the MX record for their domain to make your server to be it's
2) set you server to allow mail relaying to that domain if you have
restrictions in place.
> I know how to set up the DNS MX records, but the problem
> is the postoffice setup?
It's both if relaying secutiry is enabled on PO, MX only if it is not