[Exim] whitelisting a dynamic host

I want to start blocking dynamic ip addresses since they have become
such a big source of spam. However, I have one friend who's mail
server is stuck on a dynamic ip address and I don't wish to block
him. Is there a simple and secure method to allow his mail server
through without opening up a big hole?

Rossz
--
Fogalmam sincs, mi van ide írva.