You can use the smtp_generic_maps of postfix to rewrite email headers for outgoing smtp mail:
user:~$ echo "http@example.com admin@example.com" >> /etc/postfix/generic
user:~$ echo "smtp_generic_maps = hash:/etc/postfix/generic" >> /etc/postfix/main.cf
user:~$ postmap /etc/postfix/generic
user:~$ service postfix restart
