For those of you with the exim, if its not sending mail with the -i option and you cant easily change this, you might want to check out the imap_mail() function which works almost exactly the same and doesnt use exim, most web hosts provide this. If you using your own server then php needs to be compiled with imap libraries etc.
See http://php.net/manual/en/function.imap-mail.php