I haven't checked my logs, but I know plenty of qpsmtpd users find
this to be one of the single most successful spam/virus filters.
It's amazingly effective, especially if coupled with a longer delay for
hosts
that are on a spam blacklist. I delay hosts that are on a few dynamic IP
and
spam blacklists, or that have no reverse DNS, for 40 seconds before the
220
greeting. Any RFC-compliant mailer will have no problem with this, but a
lot
of spam software starts blissfully sending away before the delay is up.
Those
connections are sent a 5xx response and dropped.
Since most probably some spammers are members of this mailing list, I would
not disclose here too many tips how to block spam.
Roger