mail filtering at the MTA is a must to avoid congestion and overloading of
"downstream" servers ... large ISPs like AOL (where I was Internet Product
Manager) implement MTA-level filtering out of necessity: a significant portion
of their traffic (ranging 5-50%) is spam or spam-related (responses, etc)...
not that existing filtering is helping that much ... :)
the only reason you can really 'filter' spam now is that the
spammers are pretty lazy and generate obviously bogus messages...
but they're getting smarter. filtering could help in the long
term only if some kind of labelling were mandatory.