Mark as spam in a big system (GMail, Hotmail, DCC, whatever). Seriously, it doesn't take that many pissed GMail users to cause GMail to drop all of your mail. This is one reason why outfits like MailChimp will kick you off if you get "marked as spam" too often.
(You could argue that such mail isn't "real" spam; nonetheless, "mark as spam" is realistic.)
(You could argue that such mail isn't "real" spam; nonetheless, "mark as spam" is realistic.)