Configure SMTP Server in Ubuntu to run PHP script on attached files.

Hi, I have an Ubuntu 10 server running on the Amazon Cloud(EC2), and I'd like to use the SMTP server. However, I'd like the SMTP server to process any messages sent through it with an attachment, and run a PHP script (which I'll generate, it basically does some MP3 format conversions) and then send the email with the NEW MP3 attached. Any ideas? Also, I've heard rumors about SMTP servers from the AWS frequently being tagged as SPAM, so I'd love to hear any info about that if you have it, if not, no worries, I could always host the SMTP server elsewhere if it's a problem. Thanks, P.S. Please include the phrase I AM HUMAN, so that I know that you are ;)