I'm not positive how you have your form set up to email, but I use this for mine and when I click 'reply' it replies it to the email provided. Feel free to take, modify, and implement as you feel the need:
PHP Code:
<?php
$ini = parse_ini_file("includes/config.ini.php");
$to = $ini['adminEmail'];
$subject = $ini['emailSubject'];
$headers = array();
$headers[] = "MIME-Version: 1.0";
$headers[] = "Content-type: text/plain; charset=iso-8859-1";
$headers[] = "From: Transcendence <" . $ini['adminEmail'] . ">";
$headers[] = "Reply-To: " . $_POST["username"] . " <" . $_POST["email"] . ">";
$headers[] = "Subject: {$subject}";
$headers[] = "X-Mailer: PHP/" . phpversion();
$message = "Greetings <B>" . $ini['administrator'] . "</B>,\r\n\r\nA user has requested access to the Administrative Control panel via the Transcendence page.\r\n\r\nThe Users Info is as Follows:\r\n\r\n<B>Username</B>: <I>" . $_POST["username"] . "</I>\r\n" . "<B>Email</b>: <I>" . $_POST["email"] . "</I>\r\n" . "<B>IP Address</B>: <I>" . $_SERVER['REMOTE_ADDR'] . "</I>\r\n" . "<B>Hashed Password</B>: <I>" . $theHash . "</I>\r\n\r\nConfirm with the User that they have requested this access. If they have, please go to the .htpasswd file and uncomment their hashed password to give them access. Thank you!\r\n\r\n\r\nTranscendence\r\n" . $ini['siteName'] . " at It's Finest";
$message = wordwrap($message, 70, "\r\n");
mail($to, $subject, $message, implode("\r\n", $headers));
?>