[exim] rate limit web user

Hi I am looking for an example to rate limit the user the web server runs under to keep under control any vulnerable email forms.


for example

2009-07-20 17:26:43 1MSvhL-0000Xv-NK <= webuser@??? U=webuser P=local S=990 id=5e1d047c150d000ece8197c3287d8315@??? T="Llanelly: Search Engine Optimization"
2009-07-20 17:26:45 1MSvhL-0000Xv-NK => support@??? R=dnslookup T=remote_smtp H=mx01.1and1.co.uk [212.227.15.169]
2009-07-20 17:26:45 1MSvhL-0000Xv-NK Completed

will have been submitted from a php mail form.
I have increased logging for starters so i can track at least if there is a problem which domain it has come from.
So I would like to rate limit webuser@???.
Once that's done the next thing would be to sort out some sort of warning so I know its going on rather than find out later I have been responsible for thousands of spam and got my self black listed
Any info welcome