[Exim] TARPIT ACL - for Spam Control

I'd like to propose a new feature to be added to Exim as part of ACLs.
In addition to ACCEPT and DENY I'd like to see TARPIT which is to accept
- but to do so at a very slow rate. And - there would have to be other
commands to define what a slow rate means.

The reson I think this would be worthwhile is to help control spam. Not
only to protect the server from overload - but to be able to slow down
spammers to the point where they are delivering far less spam. I don't
know if this will work - but I think it's worth finding out. And - if
it's as easy (easy is very important) as using TARPIT instead of ACCEPT
that a lot of people will try it.

Example:

TARPIT dnslists = blackholes.mail-abuse.org

I believe this would be effective in that hard core spammers are running
at 100% of their capacity. Once they deliver their list they start over
and resend it. If they were slowed by 50% then they could send 50% less
spam.

OTOH, often legitimate email is blocked by accidentally being on a
blacklist and free speech is often infringed accidentally by wrongly
blocking hosts. I've done it myself and it is embarrasing when it
happens. If a legitimate newsletter were wrongly tarpitted - which will
happen to someone - it's no big deal. The newsletter will get delivered
- just more slowly. Since the newsletter isn't trying to saturate it's
capacity with spam - a slowdown isn't something that will even be that
noticeable. So - it's my theory that it's something that will hurt
spammers only.

So - I'm proposing that TARPIT be added to Exim and let's try it out.