验证域名发送电子邮件

How can I prove to email clients that the sent email was sent from the domain which the email was sent from? I have included the code I use to send emails.

$headers = "From: noreply@mydomain.com
";
$headers .= "Reply-To: noreply@mydomain.com
";
$headers .= "MIME-Version: 1.0
";
$headers .= "Content-Type: text/html; charset=ISO-8859-1
";  
$to = $_GET['to'];
$subject = $_GET['subject'];
$message = $_GET['message'];
$headers = "From:" . $from;
mail($to,$subject,$message,$headers);