There is a good change that the foreign email servers will drop if the sender email headers are not properly formatted.
Anyway if you get apache@localhost in Received header (with postfix/apache/php), you can change the php.ini, sendmail_path program to use sendmail.postfix instead of sendmail binary.
like : sendmail_path = /usr/sbin/sendmail.postfix -t -i -f
./arun