How to determine which PHP-script allows spamming?

Hello,

I have a big problem. Some customer probably got installed a PHP-script 
that allows to send-out mails with no trace to the original domain it 
belongs to (we had this before, were pollvote.php was used to install 
some kind of web-shell - but it was easily detectable which domain it was).

The problem is that I have close to 10000 domains  on my cluster.
I tried to correlate httpd-logs with the maillogs, but it didn't lead to 
anything useful.
I'm currently grep'ing the whole content for some of the email-addresses 
used, but I'm pessimistic - it may be that the spammer loads even that 
list from remote - and it takes a lot of time to grep 400 GB.


What options do I have?
Can Snort detect this?

(The webserver uses qmail as MTA)



cheers,
Rainer