Frequent contributor Bennett Haselton writes: "Hotmail and Yahoo Mail are apparently sharing a secret blacklist of domain names such that any mention of these domains will cause a message to be bounced back to the sender as spam. I found out about this because — surprise! — some of my new proxy site domains ended up on the blacklist. Hotmail and Yahoo are stonewalling, but here's what I've dug up so far — and why you should care." Read on for much more on how Bennett figured out what's going on, and why it's a hard problem to solve.