I don't have much of a problem about Abovenet having a policy of not allowing spam across their network, as you say it is theirs. What is bad, however, is that Abovenet still advertise the prefix surrending hosts they block, blackholing these hosts for their BGP mutilhomed customers -

this is pretty hard for abovenet to fix, since no router i'm aware of is able to take in a /13 from some customer, blackhole a smattering of /32's inside of it, and only send the remnants of the /13 to its downstreams. what MIBH does, as a transit customer of abovenet among others, is to take an RBL BGP feed just for the purpose of route-mapping all of its contents toward one of our other transit providers. (MIBH is the transit provider for MAPS, among other things, and MAPS has web servers which *must* be able to be reached by people listed on its RBL. is that funny, or what?) i agree that this is painful, and if it were my network i'd find some way to let abovenet's multihomed customers automatically avoid abovenet for things which aren't going to work.