Index: src/applications/differential/editor/DifferentialTransactionEditor.php
===================================================================
--- src/applications/differential/editor/DifferentialTransactionEditor.php
+++ src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -755,6 +755,12 @@
     return PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix');
   }
 
+  protected function getMailThreadID(PhabricatorLiskDAO $object) {
+    // This is nonstandard, but retains threading with older messages.
+    $phid = $object->getPHID();
+    return "differential-rev-{$phid}-req";
+  }
+
   protected function buildReplyHandler(PhabricatorLiskDAO $object) {
     return id(new DifferentialReplyHandler())
       ->setMailReceiver($object);