[Spambayes] pushing back the cost of spam

[Neil Schemenauer]

Here's an idea.  Do spam filtering at the transport level (i.e. "STMP
time").  When a message is considered spam by the filter, return a
temporary error (i.e. 4xx).  Include the number of times the message
delivery has been retried and the time since the first attempt as part
of the evidence when filtering.  RFC 2821 specifies that messages should
be retried for at least 4 days.  If the message is still being retried
after, say, 2 days and is still flagged as spam by the filter then
accept it but save it in the spam folder.

I think if this system was widely implemented the spammer's job would
become considerably more difficult.  Spammers rely on hit and run
tactics and I don't think they could tolerate a one or two day delay.
Abused open relays would become heavily loaded due to all messages
queued and the retries.  When the open relay was secured hopefully the
queue of spam would be cleared.  Also, I believe most email viruses do
not retry after a temporary error.

Finally, I'm guessing that a retry after one day would end up being a
strong ham clue.  Legitimate email that was initially considered spam
would have a better chance of not ending up in the spam folder.

Thoughts?

  Neil