PHP's mail function doesn't send email
<p>So when I submit my form, everything is stored in the database just fine, I had a really simple email that basically consisted of, 'Hey thanks, this is your link' kind of deal and I received that just fine. But now I've upgraded to using an HTML body instead and suddenly I'm not receiving the email, any idea why?</p>

<pre><code>$to = $username;
$subject = 'Thanks for Registering on Dota Pub Stars!';
$message = '&lt;html&gt;
                &lt;head&gt;
                    &lt;title&gt;Registration Successful!&lt;/title&gt;
                    &lt;link rel="stylesheet" src="http://www.sithhappens.net/css/style.css" /&gt;
                &lt;/head&gt;
            &lt;body class="bodybg"&gt;
                &lt;div id="registrationform"&gt;
                &lt;p class="success"&gt;Thanks for registering on Dota Pub Stars!&lt;/p&gt;
                &lt;br /&gt;
                &lt;br /&gt;
                &lt;a class="button2" href="http://www.sithhappens.net/?key=' . $activation_key . '&amp;amp;username=' . $username . '&amp;amp;email=' . $email1 . '"&gt;Activate Account&lt;/a&gt;
                &lt;/div&gt;
            &lt;/body&gt;
            &lt;/html&gt;';

$headers = "MIME-Version: 1.0 \r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\n";
$headers .= "From: $from\r\n";

mail($to, $subject, $message, $headers);
</code></pre>
