E-Mail::Spam::RBL's Bad ?
There is a long article at http://theory.whirlycott.com/~phil/antispam/rbl-bad/rbl-bad.html discussing alternatives to using Real Time Black List's (RBL's) to block delivery of messages from mail servers known to allow open relay.
I haven't read the entire thing, but I think--so far--I disagree with most of it.
I don't use RBL's for some of these reasons, but I respect them in principle and certainly wouldn't attack the ideas or their implementations as being unconstitutional, censorship, or discriminatory. A lot of those points seem to really be stretching it.
Let me know what you think