Newsletter sign up email validation

I am using a form to get newsletter sign ups on my website. I am using a contact.php file which works well but there is no validation so I occasionaly and sometimes frequently get blank responses.

I'm not sure why this is, but I believe I need validation.

This is my original code

<?php
/*

Author: Andrew Walsh
Date: 30/05/2006
Codewalkers_Username: Andrew


This script is a basic contact form which uses AJAX to pass the information to php, thus making the page appear to work without any refreshing or page loading time.

*/

$to = "hello@interzonestudio.com"; //This is the email address you want to send the email to
$subject_prefix = ""; //Use this if you want to have a prefix before the subject

if(!isset($_GET['action']))




{
die("You must not access this page directly!"); //Just to stop people from visiting contact.php normally
}

/* Now lets trim up the input before sending it */


$subject = "Newsletter Sign Up"; //The senders subject
$message = trim($_GET['email']); //The senders subject
$email = trim($_GET['email']); //The senders email address

mail($to,$subject,$message,"From: ".$email.""); //a very simple send

echo 'contactarea|Thank you. We promise you won’t regret it.'; //now lets update the "contactarea" div on the contact.html page. The contactarea| tell's the javascript which div to update.
?>

and this is the code I tried to add to validate but it doesnt work.

<?php
/*

Author: Andrew Walsh
Date: 30/05/2006
Codewalkers_Username: Andrew


This script is a basic contact form which uses AJAX to pass the information to php, thus making the page appear to work without any refreshing or page loading time.

*/

$to = "jcash1@gmail.com"; //This is the email address you want to send the email to
$subject_prefix = ""; //Use this if you want to have a prefix before the subject

if(!isset($_GET['action']))




{
die("You must not access this page directly!"); //Just to stop people from visiting contact.php normally
}

/* Now lets trim up the input before sending it */


$subject = "Newsletter Sign Up"; //The senders subject
$message = trim($_GET['email']); //The senders subject
$email = trim($_GET['email']); //The senders email address


/* Validation */
$error=0; // check up variable
$errormsg = '<ul class="errorlist">';

/* get it checking */


if(!check_email($email))
{
  $errormsg.=  "<li class='errormessage'>ERROR: not a valid email.</li>";
  $error++;
}

$errormsg .= '</ul>';


if($error == 0) {


mail($to,$subject,$message,"From: ".$email.""); //a very simple send

echo 'contactarea|Thank you. We promise you won’t regret it.'; //now lets update the "contactarea" div on the contact.html page. The contactarea| tell's the javascript which div to update.

 } else { 
    echo 'error|'. $errormsg;
 }

?>

Can anyone offer some insight?