[exim] Exim as a relay - ratelimiting by rejections

Hello,

We're an ISP and use exim as a smarthost/relay for our customers.

We are trying to cut down the amount of spam we relay. We currently run
messages through clamd with extra signatures from Sanesecurity (Thanks
Steve), and stop customers mail servers relaying bounces through us to
cut down backscatter.

I don't really want to run all mail through spamassassin as that will be
expensive.

However, is there a way to ratelimit or deny hosts based on how many
messages they've sent that have been rejected/deferred by the recipients MX?

Eg, if we've tried to relay messages for a host, and 100 in the past
hour have been denied or deferred by the recipients MX, then we can
block the sender from sending more. (as well as log it and contact the
customer)

Thanks.

--
Andrew