MX checking is as broken or more broken than bayes.
I have to say, OSDL has been doing MX checking, and it's effective as hell. Most importantly, when it _does_ break, it's not because some "content" is considered inappropriate, it's because some ISP does something technically wrong.
OSDL also refused to talk to open mail relays etc. I got into something of a (fairly civilized) shouting match with John Gilmore over it, who used to send out email from a "fake open mail relay" on princuple (maybe he still does). He claimed I was censoring his free speech rights when I didn't read his emails, but I just told him that I was expressing my right to not listen to people who are so stupid that they can't configure their email servers.