diff -ura subscribe-to-comments.o/subscribe-to-comments.php subscribe-to-comments/subscribe-to-comments.php
--- subscribe-to-comments.o/subscribe-to-comments.php	2008-10-18 12:15:57.000000000 +0200
+++ subscribe-to-comments/subscribe-to-comments.php	2008-10-18 15:13:38.000000000 +0200
@@ -158,6 +158,14 @@
 
         echo '<li><label for="author_text">' . __('Entry Author', 'subscribe-to-comments') . '</label><br /><textarea style="width: 98%; font-size: 12px;" rows="2" cols="60" id="author_text" name="sg_subscribe_settings[author_text]">' . sg_subscribe_settings::textarea_setting('author_text') . '</textarea></li>';
 
+        echo '</ul></fieldset>';
+
+        echo '<fieldset><legend>' . __('E-Mail Text', 'subscribe-to-comments') . '</legend>';
+
+        echo '<ul>';
+
+        echo '<li><label for="double_opt_in">' . __('Double Opt-In', 'subscribe-to-comments') . '</label><br /><textarea style="width: 98%; font-size: 12px;" rows="10" cols="60" id="double_opt_in" name="sg_subscribe_settings[double_opt_in]">' . sg_subscribe_settings::textarea_setting('double_opt_in') . '</textarea></li>';
+
         echo '</ul></fieldset>';
 
 
@@ -413,18 +421,89 @@
         global $wpdb;
         $cid = (int) $cid;
         $id = (int) $id;
-        $email = strtolower($wpdb->get_var("SELECT comment_author_email FROM $wpdb->comments WHERE comment_ID = '$cid'"));
+
+        $row = $wpdb->get_row("SELECT comment_author_email, comment_post_ID, comment_author_IP from $wpdb->comments WHERE comment_ID = '$cid'");
+
+        $email = $row->comment_author_email;
+        $postid = $row->comment_post_ID;
+        $ip = $row->comment_author_IP;
+
         $email_sql = $wpdb->escape($email);
-        $postid = $wpdb->get_var("SELECT comment_post_ID from $wpdb->comments WHERE comment_ID = '$cid'");
 
         $previously_subscribed = ( $wpdb->get_var("SELECT comment_subscribe from $wpdb->comments WHERE comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email_sql' AND comment_subscribe = 'Y' LIMIT 1") || in_array($email, (array) get_post_meta($postid, '_sg_subscribe-to-comments')) ) ? true : false;
 
         // If user wants to be notified or has previously subscribed, set the flag on this current comment
         if (($_POST['subscribe'] == 'subscribe' && is_email($email)) || $previously_subscribed) {
-            delete_post_meta($postid, '_sg_subscribe-to-comments', $email);
-            $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'Y' where comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email'");
+            delete_post_meta($postid, '_sg_subscribe-to-comments', $email);
+
+            $sendOptIn = true;
+            if ($previously_subscribed) {
+                $sendOptIn = false;
+            }
+
+            if ($sendOptIn) {
+                // check if author has subscribed to an comment in the last 3 months
+
+                $sql = "SELECT COUNT(*)
+                          FROM " . $wpdb->comments . "
+                         WHERE DATE_SUB(CURDATE(), INTERVAL 3 MONTH) <= comment_date_gmt
+                           AND LCASE(comment_author_email) = '" . $email_sql . "'
+                           AND comment_subscribe = 'Y'";
+
+                $result = $wpdb->get_var($sql);
+                if ($result >= 1) {
+                    $sendOptIn = false;
+                }
+            }
+
+            if ($sendOptIn) {
+                // send double-opt in
+                $key = md5($cid . $email . $postid . $ip);
+
+                // link
+                $link  = get_option('home') . '/?wp-subscription-manager=1&opt-in=1';
+                $link = add_query_arg('cid', $cid, $link);
+                $link = add_query_arg('key', urlencode($key), $link);
+
+                $message = isset($this->settings['double_opt_in']) ? $this->settings['double_opt_in'] : "Click to confirm:\n[link]";
+
+                $message = str_replace("[link]", $link, $message);
+
+                $this->send_mail($email, "Confirm the subscription ", $message);
+            } else {
+                $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'Y' where comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email'");
+            }
         }
         return $cid;
+    }
+
+    function add_opt_in_subscriber($cid, $key)
+    {
+        global $wpdb;
+
+        $cid = (int) $cid;
+        $row = $wpdb->get_row("SELECT comment_author_email, comment_post_ID, comment_author_IP, comment_subscribe from $wpdb->comments WHERE comment_ID = '$cid'");
+
+        if (!is_object($row)) {
+            return 1;
+        }
+
+        if ($row->comment_subscribe == "Y") {
+            return 2;
+        }
+
+        $postid = $row->comment_post_ID;
+        $email = $row->comment_author_email;
+
+        $checkkey = md5($cid . $email . $postid . $row->comment_author_IP);
+
+        if ($key != $checkkey) {
+            return 3;
+        }
+
+        $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'Y' where comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email'");
+
+        return 0;
     }
 
 
@@ -860,9 +939,34 @@
 }
 
 function sg_subscribe_admin($standalone = false) {
-    global $wpdb, $sg_subscribe;
-
-    sg_subscribe_start();
+    global $wpdb, $sg_subscribe;
+
+    sg_subscribe_start();
+
+    if (isset($_REQUEST["opt-in"], $_REQUEST["cid"], $_REQUEST["key"])) {
+        $result = $sg_subscribe->add_opt_in_subscriber($_REQUEST["cid"], $_REQUEST["key"]);
+
+        switch ($result) {
+
+            case 0:
+                die ( __('Successfully subscribed!', 'subscribe-to-comments') );
+                break;
+
+            case 1:
+                die ( __('Error while fetching db record!', 'subscribe-to-comments') );
+                break;
+
+            case 2:
+                die ( __('Already subscribed!', 'subscribe-to-comments') );
+                break;
+
+            case 3:
+                die ( __('Invalid key!', 'subscribe-to-comments') );
+                break;
+
+        }
+        die ( __('You may not access this page without a valid key.', 'subscribe-to-comments') );
+    }
 
     if ( $standalone ) {
         $sg_subscribe->form_action = get_option('home') . '/?wp-subscription-manager=1';