[exim] Drop messages at RCPT time based on IP/Subnet

Hi Everyone,

Is there a way to knock back connections from an IP address/subnet at
receipt time (in the same fashion that a connection is rejected if a
match is found in a dnsbl)?

I am ultimately wanting to refuse mail from a subnet:

ITECH SERVICES, LLC CUST-NETBLK-PHX-23-231-32-0-21 (NET-23-231-32-0-1)
23.231.32.0 - 23.231.39.255

Keep getting spam from IP's on this subnet. The problem is that by the
time individual IP addresses end up in the likes of zen.spamhaus,
bl.spamcop etc, they have already been able to drop off their payload.

I have the option of firewalling out the IP range, but I would prefer to
do it at MTA level so things are logged (useful in case legit mail gets
dropped so I can identify if I am being too heavy handed).

It wound be nice to be able to reject connections from say a /24 and
have the server reply back with "550: Sorry too much spam from your
network"

Cheers,
Alex.