One approach that I've thought casually revolves around a feature in
Sendmail that would open an SMTP connection back to the sending host
(assuming the sender had smtp services) and test it for relaying mail back
to you. You could then accept or reject mail from that host based on the
finding.
won't work. lots of sites have separate outgoing and incoming MTAs, and
nothing says that a 'sending host' even has to listen for inbound SMTP
connections.
In fact for really huge email setups this is the rule, not the exception.
admittedly, lots of other ideas for filtering spam - some that people
actually implement, like filtering out source addresses that correspond
to dialup lines - are no better.
I agree that this sort of filtering is wrong. However, in the overall
scheme of things it doesn't break things nearly as badly as what's proposed
above.
Ned