Re: [9fans] re: spam filtering fs

[Geoff Collyer <geoff@collyer.net>]

Knocking at the gate is one thing, crashing through with a bus full of
aggressive salesmen in loud checks and plaids is another.  A scheme
that seems to be closer to what Ron wants would be to have the sending
system contact the receiving system and announce that user so-and-so
has a message of this size with this message-id that he wants to send
to user thus-and-such on the receiving system; possibly there's some
form of authentication that it really is so-and-so sending it.  If
thus-and-such's white-list permits it (presumably based on the sending
user), the message is let through; if his black-list denies it, the
message is refused, and if neither list mentions the sender, the
sending system is told `we'll get back to you', the message is not
accepted, and the receiving system appends a brief notification to
thus-and-such's gray-list.  Periodically, maybe once a day,
thus-and-such scans his gray-list and if he sees a message that he
wants to look at it, he fetches it by message-id from the sending
system, incurring the band-width expenditure then.

This could almost be cobbled together today using SMTP and POP3 or
(god help us) IMAP4.  The major change would be to have the sending
system hang on to unsent (gray-listed) messages for some period so
that they may be retrieved.  I suppose that could almost be
accomplished by having smtpd return the cabbalistic SMTP return code
meaning `try again later'.