[exim] Defer as a spam filter

Hi all, we recently had a presentation from Mirapoint on how, if we
payed them oodles of dosh, they could install some really expensive
boxes and sell us some really expensive software to do exactly what we
do with exim, spamassassin and clamav. One of the things that I did find
interesting (or even scary) was their explanation of how something they
call mail-hurdle works as a spam filter. Basically it is this.

When an MTA tries to deliver an email, the sender (they were not sure if
it was the actual sender email address or the sending server helo) is
looked up in a database, if they are valid senders then the email is
accepted. If they are not in the database, then the message is given a
defer, the sender is then written into the database. The idea is when
they retry they will be in the database as so they accept the email. The
whole point being that genuine email will retry, spammers do not retry.
Now we could implement this with exim, perl and mysql fairly easly. But
wondered what people thought of it as a policy.

Personally I think it is bad etiquette and unfair on the sending MTA,
but on the other hand as most of the MTAs you deal with a lot will be in
the database it should not be too much of a problem.

I would appreciate any feedback on this as some people think we should
spend squillions of pounds on this 'professional' system rather than our
free system which is IMHO infinitely better.

Ron
--
Ron McKeating
Senior IT Services Specialist
Computing Services
Loughborough University
01509 222329