[exim] gathering stats about spam plus help with filtering o…

Hi,

I'd like to gather some stats about the proportion of spam going thru
our servers. In the past we have not implemented spam-filtering as we
are more of a carrier for our customers; we have very few mailboxes.
However I'm convinced that our servers would breathe more easily if they
didn't have to process so much spam. Can I use spam assassin (or another
program) as a stand alone program to analyse what proportion of spam is
in my mail spool? Could you suggest other ways of doing this?

Also what options are available for limiting mail coming from a certain
host? This needs to be done automatically. Ideally if a number of
messages from a certain host exceeds a threshold I want to do something
like reject them or divert them to another server, just get them off the
main queue. I have an external script that analyses the log and writes
the names of domains that have got too many mails in the queue to a
file. A router in exim reads this file and passes these messages to
another server. It works but I'm not sure it's the best, like is there a
performance hit reading the file containing the name of domains to be
moved? Also the domain is only removed once there is already a problem
with the spool, I'd like to catch problems a bit quicker. A main aim
behind this is to reduce the amount of manual effort in keeping exim
running smoothly. Possibly use smtp_ratelimit_hosts =
partial-lsearch;/usr/exim/ratelimited_host if the file
'ratelimited_hosts' could be updated automatically - but how?

Hope someone can help.

cheers,
Martin