[exim] Cheap Fast Grey List Idea

My own grey listing is far to complex for posting here but with 4.68 and
the new noupdate feature in ratelimiting I'd like to suggest hat a
simple greylisting system can be implemented. Here's what I propose.

You have 3 IP addresses on the same server and 3 MX records. The idea is
that if you touch the lowest 2 MX records you get a 421 error the first
time but after that you are cleared for one day. So if the hit the
lowest MX then it returns 421 DEFER. But when they retry on the second
MX then it accepts. As long as the host sends and email once a day there
is not DEFER. If it's longer that once a day then you get a defer once.

The idea of having 2 good MX records is that most MTAs (except Qmail)
will immediately retry on the next higher MX. In the case of qmail the
server will retry in it's normal cycle so the message might be delayed.

The third highest MX always returns DEFER 421 and sets another counter
that locks the host out for say 30 minutes and all interfaces. Thus spam
bot traffic would be greatly reduced.

It should be really easy to code this up. But just throwing it out there
to see what people think.

If you have real backup servers you might configure your MX as follows:

10 pirmary server
20 primary server but different IP
30 secondary server
40 primary server but returns DEFER

And you can further reduce botnet spam by adding more fake MX servers on
the high end.

Who likes this idea?