Comments, now that it works.

lib/Horde/Mime/Headers/ThreadIndex.php

@@ -1,6 +1,25 @@
 <?php
 /**
+ * Copyright 2023 Horde LLC (http://www.horde.org/)
  *
+ * See the enclosed file LICENSE for license information (LGPL). If you
+ * did not receive this file, see http://www.horde.org/licenses/lgpl21.
+ *
+ * @category  Horde
+ * @copyright 2023 Horde LLC
+ * @license   http://www.horde.org/licenses/lgpl21 LGPL 2.1
+ * @package   Mime
+ */
+
+/**
+ * This class represents the (non-standard) ThreadIndex header sent by
+ * the desktop MS Outlook products.
+ *
+ * @author    Michael J. Rubinsky <mrubinsk@horde.org>
+ * @category  Horde
+ * @copyright 2023 Horde LLC
+ * @license   http://www.horde.org/licenses/lgpl21 LGPL 2.1
+ * @package   Mime
  */
 class Horde_Mime_Headers_ThreadIndex extends Horde_Mime_Headers_Element_Single
 {
@@ -18,12 +37,19 @@ public static function getHandles()
      */
     protected function _sendEncode($opts)
     {
+        // First try to encode it the standard way. This
+        // will catch any cases where the header may not contain
+        // the expected base64 encoded string, but rather some
+        // other data that would normally get encoded.
         $encoded = Horde_Mime::encode($this->value, $opts['charset']);
-
         if ($encoded != $this->value) {
             return array($this->value);
         }
 
+        // We didn't encode it already, check for extra-long
+        // headers that exceed the 998 character maximum imposed
+        // by RFC 2045. The above encoding would not normally catch
+        // this sicne the expected data does not contain any whitespace.
         $len = strlen($this->value);
         $crlf = 0;
         $needs_encoding = false;
@@ -32,7 +58,8 @@ protected function _sendEncode($opts)
             switch ($chr) {
             case 0:
                 // NULLs not valid here, should have
-                // been caught above.
+                // been caught above. Just nuke the header
+                // in this case, it's broken.
                 return array('');
             case 10:
             case 13:
@@ -47,6 +74,8 @@ protected function _sendEncode($opts)
             }
         }
 
+        // If we need encoding, encode it with RFC 2045 mime encoding
+        // then fold the resulting lines.
         if ($needs_encoding) {
             $delim = '=?' . $opts['charset'] . '?b?';
             $parts = explode(