edit /etc/postfix/master.cf


smtp inet n - - - - smtpd -o content_filter=my_scripts:
my_scripts unix - n n - - pipe
flags=Xhq user=username argv=/usr/bin/php /my_scripts.php

X - perform final delivery
h - set the domain & recipient to lowercase
q - quote white space & other special characters


more info
incoming mail and php
setup postfix to forward incoming email to php