diff --git a/mod/forum/classes/task/send_user_notifications.php b/mod/forum/classes/task/send_user_notifications.php
index 042b9bc330d..1190f66d1fc 100644
--- a/mod/forum/classes/task/send_user_notifications.php
+++ b/mod/forum/classes/task/send_user_notifications.php
@@ -485,6 +485,11 @@ class send_user_notifications extends \core\task\adhoc_task {
             } else {
                 $headers[] = "References: $parentid";
             }
+        } else {
+            // If the message IDs that Moodle creates are overwritten then referencing these
+            // IDs here will enable then to still thread correctly with the first email.
+            $headers[] = "In-Reply-To: $rootid";
+            $headers[] = "References: $rootid";
         }
 
         // MS Outlook / Office uses poorly documented and non standard headers, including