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	2008-10-20 14:19:20.000000000 +0200
@@ -158,6 +158,16 @@
 
         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_subject">' . __('Double Opt-In Subject:', 'subscribe-to-comments') . ' <input type="text" size="40" id="name" name="sg_subscribe_settings[double_opt_in_subject]" value="' . sg_subscribe_settings::form_setting('double_opt_in_subject') . '" /></label></li>';
+
+        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 +423,129 @@
         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);
+
+            $subscribe = true;
+
+            $sendOptIn = true;
+            if ($previously_subscribed) {
+                $sendOptIn = false;
+            }
+
+            if ($sendOptIn) {
+                // check if author has subscribed to a 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) {
+                // check if we already sent a mail in the last 24 hours
+
+                $sql = "SELECT COUNT(*)
+                          FROM " . $wpdb->comments . "
+                         WHERE DATE_SUB(CURDATE(), INTERVAL 1 DAY) <= comment_date_gmt
+                           AND LCASE(comment_author_email) = '" . $email_sql . "'
+                           AND comment_subscribe = 'C'";
+
+                $result = $wpdb->get_var($sql);
+                if ($result >= 1) {
+                    $sendOptIn = false;
+                    $subscribe = false;
+                }
+            }
+
+            if ($sendOptIn && !$this->is_blocked($email)) {
+                // 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]";
+
+                if (function_exists("str_ireplace")) {
+                    $message = str_ireplace(
+                        array("[link]", "[manager_link]"),
+                        array($link, $this->manage_link($email, false, false)),
+                        $message
+                    );
+                } else {
+                    $message = str_replace(
+                        array("[link]", "[manager_link]"),
+                        array($link, $this->manage_link($email, false, false)),
+                        $message
+                    );
+                }
+
+                $subject = isset($this->settings['double_opt_in_subject']) ? $this->settings['double_opt_in_subject'] : "Confirm the subscription";
+
+                $this->send_mail($email, $subject, $message);
+
+                $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'C' where comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email'");
+            }
+
+            if (!$sendOptIn && $subscribe) {
+                $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'Y' where comment_post_ID = '$postid' AND LCASE(comment_author_email) = '$email'");
+            } else {
+                // mail sent but not confirmed, set comment_subscribe = C
+                $wpdb->query("UPDATE $wpdb->comments SET comment_subscribe = 'C' where comment_post_ID = '$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 $wpdb->comments SET comment_subscribe = 'Y' where comment_subscribe = 'C' AND LCASE(comment_author_email) = '$email'");
+
+        return 0;
     }
 
 
@@ -725,18 +846,18 @@
             delete_option('sg_subscribe_settings');
             wp_redirect('http://' . $_SERVER['HTTP_HOST'] . add_query_arg('stcwpbug', '2'));
             exit;
-        }
+        }
 
         if ( $update )
             $this->update_settings($settings);
-
-        $column_name = 'comment_subscribe';
-        foreach ( (array) $wpdb->get_col("DESC $wpdb->comments", 0) as $column )
-            if ($column == $column_name)
-                return true;
-
-        // didn't find it... create it
-        $wpdb->query("ALTER TABLE $wpdb->comments ADD COLUMN comment_subscribe enum('Y','N') NOT NULL default 'N'");
+
+        $result = $wpdb->get_row("DESC " . $wpdb->comments . " comment_subscribe");
+
+        if (!is_object($result)) {
+            $wpdb->query("ALTER TABLE $wpdb->comments ADD COLUMN comment_subscribe enum('Y','C','N') NOT NULL default 'N'");
+        } else if (strpos($result->Type, "C") === false) {
+            $wpdb->query("ALTER TABLE " . $wpdb->comments . " MODIFY `comment_subscribe` enum('Y','C','N') NOT NULL default 'N'");
+        }
     }
 
 
@@ -860,9 +981,34 @@
 }
 
 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';