Spammers Using My Domain In Reply-To. I Hate Spammers.

Recently, I've seen a huge jump in "Undeliverable mail" messages coming through my mail server. I've double-checked that I'm not relaying and checked a couple return path IPs, so I know that it's just jerks spoofing the "Reply-To" header, but it makes me sick to have my domains in any way associated with this crap.