"Paul Smith" suggested:
BTW, the only solution to the 'keepalive' problem that I can think of other
than having varying reply codes is for the server to receive the message the
first time, reply with 450- until it decides whether to accept it or not, then
reply '450 ' to temporarily reject the message, but record identifying details
about the message (message id, envelope details, etc), then wait until the
sender tries to re-send it and then accept it straight away. (This method also
sort of merges the idea of 'grey listing', so might be even better at stopping
spam). However, the obvious drawback is that every message has to be sent
twice - which isn't good, but I can't see any other way to do it.
I am _seriously_ concerned about this suggestion.
Someone else on another list recently said that they are already doing this as a
matter of routine. They were relying on the observation that many spammers just
make one, pre-scripted, attempt to send each message, while good-guys will
attempt a re-send.
Once spammers decide that this method of defence is starting to get in their way
the obvious, easy tactic for them is to execute each spam run twice a few hours
apart and, Voila! the amount of spam which _everyone_ sees is doubled.