Well, I've just got an email sent to a Yahoo group and noticed it had been scored really highly on my spam checker... I looked through the email and couldn't find out how.
On re-reading the email I noticed their advertising signature!
All New Yahoo! Mail Tired of Vi@gr@! come-ons? Let our SpamGuard protect you. http://uk.docs.yahoo.com/nowyoucan.html
Yeah, well done, advertise a spam busting service in a signature, but word it (Vi@gr@) in such a way that spam checkers will throw the genuine message up as spam - genius!
I'd sign up and check if the SpamGuard thing they are advertising would detect their own addition to messages as spam, but I really can't be bothered!