So many problems and unsecure code

Please add nonce to your form. My list got flooded with spam.

I have made the following changes.

on line 71
$return = '<form class="sml_subscribe" method="post" id="sml_subscribe" action="/#sml_subscribe"><input class="sml_hiddenfield" name="sml_subscribe" type="hidden" value="1">'.wp_nonce_field( 'creating_email_subscription', 'email_subscription' );

on line 273

// Handle form Post
add_action('init', 'sml_post_action');
function sml_post_action(){
	global $wpdb;
	if ($_POST['sml_subscribe'] &&(isset( $_POST['email_subscription'] ) && wp_verify_nonce( $_POST['email_subscription'], 'creating_email_subscription' )  )) {
	
		$name = $_POST['sml_name'];
		$email = $_POST['sml_email'];
		if (is_email($email)) {

			$exists = mysql_query("SELECT * FROM ".$wpdb->prefix."sml where sml_email like '".$wpdb->escape($email)."' limit 1");
			if (mysql_num_rows($exists) <1) {
				$wpdb->query("insert into ".$wpdb->prefix."sml (sml_name, sml_email) values ('".$wpdb->escape($name)."', '".$wpdb->escape($email)."')");
			}
		}
	}
}