the third parameter of mail() is the headers where you can set the sender information. e.g.
<pre>
mail("nobody@example.com", "the subject", $message,
"From: webmaster@$SERVER_NAME\r\n"
."Reply-To: webmaster@$SERVER_NAME\r\n"
."X-Mailer: PHP/" . phpversion());
</pre>
also check the:
http://www.php.net/mail