AOL, Yahoo etc... are all flexing muscle right now in the "spam being rejected" arena.

AOL, used to blindly reject Known Dynamic Addresses. Now I believe the are rating SPAM, using the feedback methods built into the AOL mail stuff.

Yahoo, use public and private info to mark stuff as spam.

Me, I use Spamassassin with DCC, Pyzor, RulesDuJour and clamav. I am changing to doing the scanning at smtp time very soon. Right now I have it looping it around through the checking gauntlet before re-submitting to route properly.