locating origin of spammer

[Joseph Koening (jWeb)]

I got up this morning and discovered that someone sent some spam through
one of my servers. The messages were sent from the 'www' user on
localhost, which is leading me to think somewhere someone has an insecure
php or perl script that is allowing someone to designate the recipient,
the subject, body, etc. I know the machine is not open-relay (I tested it
to double check) and I checked to make sure no one had actually logged in.
I grepped all of apache's log files looking for sites that received hits
about the same time the mail started going out. What else can I do to find
how the mail is being sent? Thanks,

Joe