Yahoo spam
Considering the amount of spam I get which is sent from authenticated Yahoo webmail accounts, or sometimes even authenticated Yahoo SMTP accounts, I feel they at least could do much to clean up their mail sending act.
I already run Domainkeys checks on Yahoo, Ebay and Paypal (and others) and receive little forgery spam as a result - only spam sent from Yahoo's users which has received the slight scoring boost I give Yahoo Domainkeys mail.