I'm looking for a way to make Exchange 2007 only accept mail from a certain group of hosts. These hosts all use the same identifying rDNS entry.

Exchange 2003 uses a whitelist based on the reverse DNS, there we could add the hostname to the to the "Domain" field.

However, in Exchange 2007 this option seems to be removed. Is there a way to do this?

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!