> telling spammers 4xx or 5xx doesn't matter, they don't listen.
yes, but interestingly, every "smtp transport" (remote ip address who connects to your tcp/25 service) who ignores 5XX (which you can tell because they come back and try the same thing again over and over) is either a spammer or the output side of a proxy (which might be hard to detect). so it turns out that ignoring 5XX is like sending up a flare, "blackhole me!". -- Paul Vixie
