I've been using this little app for about 4 months and it works great called MailMon
http://webhosting-tools.com/view.cgi/MailMon
(I'm not affiliated with this product in any way)
On Cpanel servers there is no sendmail. /usr/bin/sendmail is just a symbolic link to /usr/bin/exim. All PHP/Perl scripts use the path to sendmail for any type of email functions, eg: mail()
What MailMon does is replace the symolic link with a Perl script that acts as a monitoring layer. It will log the username of the person sending out email, the full script location, time and how many messages have been sent out.
It logs this in two different ways:
1) Text file
2) MySQL database
It also has a feature where you can limit total outgoing email, eg restrict any client scripts to only send a maximum of 200 emails at a time. If the client goes over you can be notified via email
The script was only $60 and well worth it IMO. Two hours after I first installed the script 4 months agao I caught 2 spammers on my machine which I would have never found before. I have since closed there accounts and my system is running much better and I feel better as well.
Check it out - if you have any additional addons to this script I'd like to hear about it.
Cheers