I'm looking for a way to disable Qmail relaying for everyone, but allow one certain group of hosts to do so. These hosts all use the same identifying rDNS entry.

In Exchange 2003, Postfix, Exim and cPanel this can be achieved pretty easily. However, the only to do this with Qmail is to do this based on IP's.

The IP's however tend to change. These changes can occur at any time, and it is impossible to keep all the servers up-to-date to the new IP's.

Running a script that resolves the hostname and whitelists them accordingly is my last-resort option, but this is not fool-proof.

Does anyone know whether this is possible and if so, how?

Thanks!