[exim] Spammer tactics

This may not be new to others, but it's something I've only recently
noticed.

Zombie hosts are avoiding our per-host connection limits and dictionary
attack checks by simply disconnecting as soon as they get a DENY, then
immediately reconnecting and trying to send the same message again. They do
this around ten times in a row before giving up.

Anyone have any ideas how to combat this?

- Mark