Note you can also do this with domains, email addresses, etc.
Doing that with domains will open you up to any forged spam.
Well I didn't say it was a good idea to do it, just that you could... :)FWIW, I do some blocking and some "whitelist" accepting on domain names, and it seems to work fine... i.e., it tries to resolve the actual IP of the connecting mail server, and if it resolves to a "bad" domain, the connection is refused "properly"...