yz0rx - 6:32 am on Aug 14, 2009 (gmt 0)

You can try this in the directory:
find ./ -name *.php ¦ xargs grep -w "mail("

This will search for any PHP files containing the mail() function. From there, you can get an idea of which scripts are sending emails out.

If it's a script that allows visitors to send emails out (such as a 'email this' or 'share this'), a captcha may be needed so bots can't abuse it.