mail.php

<?php 

define( "RECIPIENT_NAME", "John Doe" );
define( "RECIPIENT_EMAIL", "john@doe.com" );
define( "EMAIL_SUBJECT", "Visitor Message" );

$success = false;

$name = isset( $_POST['name'] ) ? preg_replace( "/[^\.\-\' a-zA-Z0-9]/", "", $_POST['name'] ) : "";
$email = isset( $_POST['email'] ) ? preg_replace( "/[^\.\-\_\@a-zA-Z0-9]/", "", $_POST['email'] ) : "";
$message = isset( $_POST['message'] ) ? preg_replace( "/(From:|To:|BCC:|CC:|Subject:|Content-Type:)/", "", $_POST['message'] ) : "";

if ( $name && $email && $message ) {
  $recipient = RECIPIENT_NAME . " <" . RECIPIENT_EMAIL . ">";
  $headers = "From: " . $name . " <" . $email . ">";
  $success = mail( $recipient, EMAIL_SUBJECT, $message, $headers );
}

if ( isset($_GET["ajax"]) ) {
  echo $success ? "success" : "error";
} else {
	//add html for javascript off user
}
?>