Was that some wrappered service? Looked like tcp_wrappers.
I think a router with enough memory would be a better performer
for filtering activies at that layer.
one semi-solution would be to receive vixie's spam feed, at least it's as
precise as possible .. and less headache.
I would guess if you can ever get it turned up it is.
Affiliation for identifcation, not endorsement, etc etc etc...