In most case the "From:" header must be a valid email address on the server, thus you cannot use the email provided by the user. You will probably need to hard-code a From: address, then make the user's address a "Reply-To:" header:
$headers = "From: valid@yoursite.com\r\n";
$headers .= "Reply-To: " . $_REQUEST['email'] . "\r\n";
$headers .= "X-Mailer: PHP/" . phpversion();
$result = mail("paul@pkeen.co.uk", "Feedback Form Results",
$message, $headers);
if(!$result)
{
die("An error occurred while trying to send the email.");
}
Also, as currently implemented, this is susceptible to email header injection attacks. I would at a minimum suggest adding the following before sending the email:
if(preg_match('/[\r\n]/', $_REQUEST['email']))
{
die("Invalid email address, possible email header injection attack");
}