Hello,
Our latest release of exim (4.34) uses Jeff Lasmans new exim.conf and we've included a new exim.pl. Together they should help fight spam in both directions. There is a new feature such that you just create (or edit) /etc/virtual/limit and enter the maximum number of emails you want any user to send daily. DA will check and enforce this limit.
I don't believe DA clears out the /etc/virtual/usage directory daily, so you'd need to setup a cron to do that (so the daily usage is reset back to zero and they can keep on sending mail)
Once you get that setup, just check /etc/virtual/usage for the largest file and the name of the file will be the user who is sending out the emails.
Note: it will catch emails sent with mail() in php, so long as the scripts are stored below the users home directory.
John