People might want to look at the recently-implemented Postfix
sender verification feature in current snapshot releases.
This is only a few months old, but it does, nicely and carefully, a
lot of what people are discussing or asking for.
* In verifying the sender address, it gets as far as connecting, doing
MAIL FROM: <> and verifying a 250/5xx for the RCPT TO, then does a
RSET on the machine it is verifying it on before disconnecting.
* If it gets a 250 for the sender address, it passes that particular
test, and if the other UCE tests pass it will return a 250 acceptance
to the sender.
* If it gets a 5xx, it remembers the address as invalid, and returns a
failure to the sender.
* If it is unable to immediately validate the sender, it returns a 4xx
SMTP code requesting a retry, while it continues attempting the
validation so it will have the answer when they do retry.
I've been using exactly this (not postfix, some code I wrote) as a
means of filtering mail for at least six years now.
It used to be a nearly perfect way of blocking spam. The only 'false
positives' came from people who had broken return addresses, usually due
to mailer misconfiguration. (so it's not as if you could reply to
them and tell them to fix it...)
For the last year or so, it's been nearly useless for me.
Keith
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