For example, anything that resolves to a rr.com address please take ahike! I would love to do this too. Unfortunately, this is Road Runner which spans 24 states. Blocking it would certainly block legitimate e-mail :(
Just my $0.02 --I see your point, but how many people actually have legit mail servers in the rr.com domain? I haven't seen any yet... same for Verizon and PacBell DSL, etc. Especially for the cable users, I don't think they're "allowed" to run mail servers; all legit mail should be coming from recognizable MX servers set up by RR, etc.