diff --git a/src/applications/differential/mail/DifferentialMail.php b/src/applications/differential/mail/DifferentialMail.php
index 2f539caf50..4b607ae22f 100644
--- a/src/applications/differential/mail/DifferentialMail.php
+++ b/src/applications/differential/mail/DifferentialMail.php
@@ -96,7 +96,7 @@ abstract class DifferentialMail {
       ->setBody($body)
       ->setIsHTML($this->shouldMarkMailAsHTML())
       ->setParentMessageID($this->parentMessageID)
-      ->addHeader('Thread-Topic', $this->getRevision()->getTitle());
+      ->addHeader('Thread-Topic', $this->getThreadTopic());
 
     $template->setAttachments($attachments);
 
@@ -327,6 +327,11 @@ EOTEXT;
     return "differential-rev-{$phid}-req";
   }
 
+  protected function getThreadTopic() {
+    $phid = $this->getRevision()->getPHID();
+    return "Differential Revision {$phid}";
+  }
+
   public function setComment($comment) {
     $this->comment = $comment;
     return $this;
diff --git a/src/applications/maniphest/editor/ManiphestTransactionEditor.php b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
index 4876fb5849..d9307b3aa2 100644
--- a/src/applications/maniphest/editor/ManiphestTransactionEditor.php
+++ b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
@@ -278,7 +278,7 @@ final class ManiphestTransactionEditor {
       ->setVarySubjectPrefix("[{$action}]")
       ->setFrom($transaction->getAuthorPHID())
       ->setParentMessageID($this->parentMessageID)
-      ->addHeader('Thread-Topic', 'Maniphest Task '.$task->getID())
+      ->addHeader('Thread-Topic', 'Maniphest Task '.$task->getPHID())
       ->setThreadID($thread_id, $is_create)
       ->setRelatedPHID($task->getPHID())
       ->setIsBulk(true)