diff --git a/src/applications/audit/editor/PhabricatorAuditEditor.php b/src/applications/audit/editor/PhabricatorAuditEditor.php
--- a/src/applications/audit/editor/PhabricatorAuditEditor.php
+++ b/src/applications/audit/editor/PhabricatorAuditEditor.php
@@ -473,17 +473,14 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $identifier = $object->getCommitIdentifier();
     $repository = $object->getRepository();
-    $monogram = $repository->getMonogram();
 
     $summary = $object->getSummary();
     $name = $repository->formatCommitName($identifier);
 
     $subject = "{$name}: {$summary}";
-    $thread_topic = "Commit {$monogram}{$identifier}";
 
     $template = id(new PhabricatorMetaMTAMail())
-      ->setSubject($subject)
-      ->addHeader('Thread-Topic', $thread_topic);
+      ->setSubject($subject);
 
     $this->attachPatch(
       $template,
diff --git a/src/applications/auth/editor/PhabricatorAuthSSHKeyEditor.php b/src/applications/auth/editor/PhabricatorAuthSSHKeyEditor.php
--- a/src/applications/auth/editor/PhabricatorAuthSSHKeyEditor.php
+++ b/src/applications/auth/editor/PhabricatorAuthSSHKeyEditor.php
@@ -255,11 +255,9 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
     $name = $object->getName();
-    $phid = $object->getPHID();
 
     $mail = id(new PhabricatorMetaMTAMail())
-      ->setSubject(pht('SSH Key %d: %s', $id, $name))
-      ->addHeader('Thread-Topic', $phid);
+      ->setSubject(pht('SSH Key %d: %s', $id, $name));
 
     // The primary value of this mail is alerting users to account compromises,
     // so force delivery. In particular, this mail should still be delivered
diff --git a/src/applications/badges/editor/PhabricatorBadgesEditor.php b/src/applications/badges/editor/PhabricatorBadgesEditor.php
--- a/src/applications/badges/editor/PhabricatorBadgesEditor.php
+++ b/src/applications/badges/editor/PhabricatorBadgesEditor.php
@@ -87,12 +87,10 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $name = $object->getName();
     $id = $object->getID();
-    $topic = pht('Badge %d', $id);
     $subject = pht('Badge %d: %s', $id, $name);
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($subject)
-      ->addHeader('Thread-Topic', $topic);
+      ->setSubject($subject);
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
--- a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
+++ b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
@@ -309,13 +309,11 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $id = $object->getID();
     $name = $object->getName();
     $monogram = $object->getMonogram();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("{$monogram}: {$name}")
-      ->addHeader('Thread-Topic', $monogram);
+      ->setSubject("{$monogram}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -227,11 +227,9 @@
         '%s sent you a message.',
         $this->getActor()->getUserName());
     }
-    $phid = $object->getPHID();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("Z{$id}: {$title}")
-      ->addHeader('Thread-Topic', "Z{$id}: {$phid}");
+      ->setSubject("Z{$id}: {$title}");
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/countdown/editor/PhabricatorCountdownEditor.php b/src/applications/countdown/editor/PhabricatorCountdownEditor.php
--- a/src/applications/countdown/editor/PhabricatorCountdownEditor.php
+++ b/src/applications/countdown/editor/PhabricatorCountdownEditor.php
@@ -45,8 +45,7 @@
     $name = $object->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("{$monogram}: {$name}")
-      ->addHeader('Thread-Topic', $monogram);
+      ->setSubject("{$monogram}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -689,15 +689,10 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
     $title = $object->getTitle();
-
-    $original_title = $object->getOriginalTitle();
-
     $subject = "D{$id}: {$title}";
-    $thread_topic = "D{$id}: {$original_title}";
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($subject)
-      ->addHeader('Thread-Topic', $thread_topic);
+      ->setSubject($subject);
   }
 
   protected function getTransactionsForMail(
diff --git a/src/applications/files/editor/PhabricatorFileEditor.php b/src/applications/files/editor/PhabricatorFileEditor.php
--- a/src/applications/files/editor/PhabricatorFileEditor.php
+++ b/src/applications/files/editor/PhabricatorFileEditor.php
@@ -47,8 +47,7 @@
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("F{$id}: {$name}")
-      ->addHeader('Thread-Topic', "F{$id}");
+      ->setSubject("F{$id}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/fund/editor/FundInitiativeEditor.php b/src/applications/fund/editor/FundInitiativeEditor.php
--- a/src/applications/fund/editor/FundInitiativeEditor.php
+++ b/src/applications/fund/editor/FundInitiativeEditor.php
@@ -50,8 +50,7 @@
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("{$monogram}: {$name}")
-      ->addHeader('Thread-Topic', $monogram);
+      ->setSubject("{$monogram}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/legalpad/editor/LegalpadDocumentEditor.php b/src/applications/legalpad/editor/LegalpadDocumentEditor.php
--- a/src/applications/legalpad/editor/LegalpadDocumentEditor.php
+++ b/src/applications/legalpad/editor/LegalpadDocumentEditor.php
@@ -124,12 +124,10 @@
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
-    $phid = $object->getPHID();
     $title = $object->getDocumentBody()->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("L{$id}: {$title}")
-      ->addHeader('Thread-Topic', "L{$id}: {$phid}");
+      ->setSubject("L{$id}: {$title}");
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/macro/editor/PhabricatorMacroEditor.php b/src/applications/macro/editor/PhabricatorMacroEditor.php
--- a/src/applications/macro/editor/PhabricatorMacroEditor.php
+++ b/src/applications/macro/editor/PhabricatorMacroEditor.php
@@ -35,8 +35,7 @@
     $name = 'Image Macro "'.$name.'"';
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($name)
-      ->addHeader('Thread-Topic', $name);
+      ->setSubject($name);
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/maniphest/editor/ManiphestTransactionEditor.php b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
--- a/src/applications/maniphest/editor/ManiphestTransactionEditor.php
+++ b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
@@ -206,8 +206,7 @@
     $title = $object->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("T{$id}: {$title}")
-      ->addHeader('Thread-Topic', "T{$id}: ".$object->getOriginalTitle());
+      ->setSubject("T{$id}: {$title}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -1262,6 +1262,11 @@
       $headers[] = array('X-Phabricator-Must-Encrypt', 'Yes');
     }
 
+    $related_phid = $this->getRelatedPHID();
+    if ($related_phid) {
+      $headers[] = array('Thread-Topic', $related_phid);
+    }
+
     return $headers;
   }
 
@@ -1309,6 +1314,7 @@
       'Precedence',
       'References',
       'Thread-Index',
+      'Thread-Topic',
 
       'X-Mail-Transport-Agent',
       'X-Auto-Response-Suppress',
diff --git a/src/applications/owners/editor/PhabricatorOwnersPackageTransactionEditor.php b/src/applications/owners/editor/PhabricatorOwnersPackageTransactionEditor.php
--- a/src/applications/owners/editor/PhabricatorOwnersPackageTransactionEditor.php
+++ b/src/applications/owners/editor/PhabricatorOwnersPackageTransactionEditor.php
@@ -46,12 +46,10 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $id = $object->getID();
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($name)
-      ->addHeader('Thread-Topic', $object->getPHID());
+      ->setSubject($name);
   }
 
   protected function buildMailBody(
diff --git a/src/applications/paste/editor/PhabricatorPasteEditor.php b/src/applications/paste/editor/PhabricatorPasteEditor.php
--- a/src/applications/paste/editor/PhabricatorPasteEditor.php
+++ b/src/applications/paste/editor/PhabricatorPasteEditor.php
@@ -72,8 +72,7 @@
     $name = $object->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("P{$id}: {$name}")
-      ->addHeader('Thread-Topic', "P{$id}");
+      ->setSubject("P{$id}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/phame/editor/PhameBlogEditor.php b/src/applications/phame/editor/PhameBlogEditor.php
--- a/src/applications/phame/editor/PhameBlogEditor.php
+++ b/src/applications/phame/editor/PhameBlogEditor.php
@@ -48,12 +48,10 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $phid = $object->getPHID();
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($name)
-      ->addHeader('Thread-Topic', $phid);
+      ->setSubject($name);
   }
 
   protected function buildReplyHandler(PhabricatorLiskDAO $object) {
diff --git a/src/applications/phame/editor/PhamePostEditor.php b/src/applications/phame/editor/PhamePostEditor.php
--- a/src/applications/phame/editor/PhamePostEditor.php
+++ b/src/applications/phame/editor/PhamePostEditor.php
@@ -61,12 +61,10 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $phid = $object->getPHID();
     $title = $object->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($title)
-      ->addHeader('Thread-Topic', $phid);
+      ->setSubject($title);
   }
 
   protected function buildReplyHandler(PhabricatorLiskDAO $object) {
diff --git a/src/applications/pholio/editor/PholioMockEditor.php b/src/applications/pholio/editor/PholioMockEditor.php
--- a/src/applications/pholio/editor/PholioMockEditor.php
+++ b/src/applications/pholio/editor/PholioMockEditor.php
@@ -112,11 +112,9 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
     $name = $object->getName();
-    $original_name = $object->getOriginalName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("M{$id}: {$name}")
-      ->addHeader('Thread-Topic', "M{$id}: {$original_name}");
+      ->setSubject("M{$id}: {$name}");
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/phortune/editor/PhortuneCartEditor.php b/src/applications/phortune/editor/PhortuneCartEditor.php
--- a/src/applications/phortune/editor/PhortuneCartEditor.php
+++ b/src/applications/phortune/editor/PhortuneCartEditor.php
@@ -123,8 +123,7 @@
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject(pht('Order %d: %s', $id, $name))
-      ->addHeader('Thread-Topic', pht('Order %s', $id));
+      ->setSubject(pht('Order %d: %s', $id, $name));
   }
 
   protected function buildMailBody(
diff --git a/src/applications/phriction/editor/PhrictionTransactionEditor.php b/src/applications/phriction/editor/PhrictionTransactionEditor.php
--- a/src/applications/phriction/editor/PhrictionTransactionEditor.php
+++ b/src/applications/phriction/editor/PhrictionTransactionEditor.php
@@ -299,12 +299,10 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $id = $object->getID();
     $title = $object->getContent()->getTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject($title)
-      ->addHeader('Thread-Topic', $object->getPHID());
+      ->setSubject($title);
   }
 
   protected function buildMailBody(
diff --git a/src/applications/phurl/editor/PhabricatorPhurlURLEditor.php b/src/applications/phurl/editor/PhabricatorPhurlURLEditor.php
--- a/src/applications/phurl/editor/PhabricatorPhurlURLEditor.php
+++ b/src/applications/phurl/editor/PhabricatorPhurlURLEditor.php
@@ -68,8 +68,7 @@
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("U{$id}: {$name}")
-      ->addHeader('Thread-Topic', "U{$id}: ".$object->getName());
+      ->setSubject("U{$id}: {$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/ponder/editor/PonderAnswerEditor.php b/src/applications/ponder/editor/PonderAnswerEditor.php
--- a/src/applications/ponder/editor/PonderAnswerEditor.php
+++ b/src/applications/ponder/editor/PonderAnswerEditor.php
@@ -57,8 +57,7 @@
     $id = $object->getID();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("ANSR{$id}")
-      ->addHeader('Thread-Topic', "ANSR{$id}");
+      ->setSubject("ANSR{$id}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/ponder/editor/PonderQuestionEditor.php b/src/applications/ponder/editor/PonderQuestionEditor.php
--- a/src/applications/ponder/editor/PonderQuestionEditor.php
+++ b/src/applications/ponder/editor/PonderQuestionEditor.php
@@ -146,11 +146,9 @@
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
     $title = $object->getTitle();
-    $original_title = $object->getOriginalTitle();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("Q{$id}: {$title}")
-      ->addHeader('Thread-Topic', "Q{$id}: {$original_title}");
+      ->setSubject("Q{$id}: {$title}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
--- a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
+++ b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
@@ -219,12 +219,10 @@
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
-    $id = $object->getID();
     $name = $object->getName();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("{$name}")
-      ->addHeader('Thread-Topic', "Project {$id}");
+      ->setSubject("{$name}");
   }
 
   protected function buildMailBody(
diff --git a/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php b/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
--- a/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
+++ b/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
@@ -196,11 +196,9 @@
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
-    $phid = $object->getPHID();
     $title = $object->getSummaryForDisplay();
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("RQ{$id}: {$title}")
-      ->addHeader('Thread-Topic', "RQ{$id}: {$phid}");
+      ->setSubject("RQ{$id}: {$title}");
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
diff --git a/src/applications/repository/worker/PhabricatorRepositoryPushMailWorker.php b/src/applications/repository/worker/PhabricatorRepositoryPushMailWorker.php
--- a/src/applications/repository/worker/PhabricatorRepositoryPushMailWorker.php
+++ b/src/applications/repository/worker/PhabricatorRepositoryPushMailWorker.php
@@ -124,7 +124,6 @@
       ->setFrom($event->getPusherPHID())
       ->setBody($body->render())
       ->setThreadID($event->getPHID(), $is_new = true)
-      ->addHeader('Thread-Topic', $subject)
       ->setIsBulk(true);
 
     return $target->willSendMail($mail);
diff --git a/src/applications/slowvote/editor/PhabricatorSlowvoteEditor.php b/src/applications/slowvote/editor/PhabricatorSlowvoteEditor.php
--- a/src/applications/slowvote/editor/PhabricatorSlowvoteEditor.php
+++ b/src/applications/slowvote/editor/PhabricatorSlowvoteEditor.php
@@ -48,8 +48,7 @@
     $name = $object->getQuestion();
 
     return id(new PhabricatorMetaMTAMail())
-      ->setSubject("{$monogram}: {$name}")
-      ->addHeader('Thread-Topic', $monogram);
+      ->setSubject("{$monogram}: {$name}");
   }
 
   protected function buildMailBody(