Very few ISPs offer spam filtering on a per-account basis (as I pointed out) - I only wish that more of them did!
You miss the point !
Per-account is as pointless as client-side, you miss a whole load of filtering opportunities.
ISPs should offer better spam filtering.
That is the point I am trying to make (with an implied suggestion that people perhaps shop around for either different ISPs or somewhere else to host their email where proper filtering does take place).
e.g. Gmail.... which I gather is pretty good since the Postini acquisition.