This is an older post, but I thought I'd contribute the answer for others. If you want to enable Spam Assassin filtering on piped mail, you can do something like this. This is for a qmail MTA on a CentOS server: 
  
 
| /usr/bin/spamassassin | php -q /var/www/vhosts/mydoman.com/httpdocs/whmcs/pipe/pipe.php
I use three support departments: info, support, and billing. I allow unregistered users to submit to info@mydomain.com. I 'POP' this address and enable aggressive Spam Assassin filtering on it. For the other addresses, I 'pipe' them using the technique above, but also require the user to be registered if not submitting from the web interface. 
  
Hope this helps.