diff --git a/include/class.mailfetch.php b/include/class.mailfetch.php
index 0887ec7c9ce059cd7b3d71256a4c50323575ce05..1aa28471ed43c50764e44a713970a1893ba0a955 100644
--- a/include/class.mailfetch.php
+++ b/include/class.mailfetch.php
@@ -666,7 +666,7 @@ class MailFetcher {
             // Fetch the original References and assign to 'references'
             if ($headers = $this->getOriginalMessageHeaders($mid)) {
                 $vars['references'] = $headers['references'];
-                $vars['in-reply-to'] = @$headers['in-reply-to'] ?: null;
+                $vars['in-reply-to'] = $headers['message-id'] ?: @$headers['in-reply-to'] ?: null;
             }
             // Fetch deliver status report
             $vars['message'] = $this->getDeliveryStatusMessage($mid) ?: $this->getBody($mid);
diff --git a/include/class.mailparse.php b/include/class.mailparse.php
index ab69f8ce2f10a97aed0bcdb2d821ddc662691dfe..f530ac50bdafdfaa6f9bedf1c74c99c9096b4b49 100644
--- a/include/class.mailparse.php
+++ b/include/class.mailparse.php
@@ -694,7 +694,7 @@ class EmailDataParser {
             // Fetch the original References and assign to 'references'
             if ($headers = $parser->getOriginalMessageHeaders()) {
                 $data['references'] = $headers['references'];
-                $data['in-reply-to'] = @$headers['in-reply-to'] ?: null;
+                $data['in-reply-to'] = $headers['message-id'] ?: @$headers['in-reply-to'] ?: null;
             }
             // Fetch deliver status report
             $data['message'] = $parser->getDeliveryStatusMessage() ?: $parser->getBody();