diff --git a/conf/default.conf.php b/conf/default.conf.php
index 6556406956..8d2218557a 100644
--- a/conf/default.conf.php
+++ b/conf/default.conf.php
@@ -145,6 +145,35 @@ return array(
   // Domain used to generate Message-IDs.
   'metamta.domain'              => 'example.com',
 
+  // When a message is sent to multiple recipients (for example, several
+  // reviewers on a code review), Phabricator can either deliver one email to
+  // everyone (e.g., "To: alincoln, usgrant, htaft") or separate emails to each
+  // user (e.g., "To: alincoln", "To: usgrant", "To: htaft"). The major
+  // advantages and disadvantages of each approach are:
+  //
+  //   - One mail to everyone:
+  //     - Recipients can see To/Cc at a glance.
+  //     - If you use mailing lists, you won't get duplicate mail if you're
+  //       a normal recipient and also Cc'd on a mailing list.
+  //     - Getting threading to work properly is harder, and probably requires
+  //       making mail less useful by turning off options.
+  //     - Sometimes people will "Reply All" and everyone will get two mails,
+  //       one from the user and one from Phabricator turning their mail into
+  //       a comment.
+  //     - Not supported with a private reply-to address.
+  //   - One mail to each user:
+  //     - Recipients need to look in the mail body to see To/Cc.
+  //     - If you use mailing lists, recipients may sometimes get duplicate
+  //       mail.
+  //     - Getting threading to work properly is easier, and threading settings
+  //       can be customzied by each user.
+  //     - "Reply All" no longer spams all other users.
+  //     - Required if private reply-to addresses are configured.
+  //
+  // In the code, splitting one outbound email into one-per-recipient is
+  // sometimes referred to as "multiplexing".
+  'metamta.one-mail-per-recipient'  => true,
+
   // When a user takes an action which generates an email notification (like
   // commenting on a Differential revision), Phabricator can either send that
   // mail "From" the user's email address (like "alincoln@logcabin.com") or
@@ -166,6 +195,7 @@ return array(
   //      mostly never arrive.
   'metamta.can-send-as-user'    => false,
 
+
   // Adapter class to use to transmit mail to the MTA. The default uses
   // PHPMailerLite, which will invoke "sendmail". This is appropriate
   // if sendmail actually works on your host, but if you haven't configured mail
@@ -329,9 +359,18 @@ return array(
 
   // Mail.app on OS X Lion won't respect threading headers unless the subject
   // is prefixed with "Re:". If you enable this option, Phabricator will add
-  // "Re:" to the subject line of all mail which is expected to thread.
+  // "Re:" to the subject line of all mail which is expected to thread. If
+  // you've set 'metamta.one-mail-per-recipient', users can override this
+  // setting in their preferences.
   'metamta.re-prefix' => false,
 
+  // If true, allow MetaMTA to change mail subjects to put text like
+  // '[Accepted]' and '[Commented]' in them. This makes subjects more useful,
+  // but might break threading on some clients. If you've set
+  // 'metamta.one-mail-per-recipient', users can override this setting in their
+  // preferences.
+  'metamta.vary-subjects' => true,
+
 
 // -- Auth ------------------------------------------------------------------ //
 
diff --git a/src/applications/audit/editor/comment/PhabricatorAuditCommentEditor.php b/src/applications/audit/editor/comment/PhabricatorAuditCommentEditor.php
index 4b27424ff8..4dd7ee3718 100644
--- a/src/applications/audit/editor/comment/PhabricatorAuditCommentEditor.php
+++ b/src/applications/audit/editor/comment/PhabricatorAuditCommentEditor.php
@@ -283,12 +283,12 @@ final class PhabricatorAuditCommentEditor {
     $reply_handler = self::newReplyHandlerForCommit($commit);
 
     $prefix = PhabricatorEnv::getEnvConfig('metamta.diffusion.subject-prefix');
-    $subject = "{$prefix} [{$verb}] {$name}: {$summary}";
+    $subject = "{$prefix} {$name}: {$summary}";
+    $vary_subject = "{$prefix} [{$verb}] {$name}: {$summary}";
 
     $threading = self::getMailThreading($commit->getPHID());
     list($thread_id, $thread_topic) = $threading;
 
-    $is_new     = !count($other_comments);
     $body       = $this->renderMailBody(
       $comment,
       "{$name}: {$summary}",
@@ -311,8 +311,13 @@ final class PhabricatorAuditCommentEditor {
     $phids = array_merge($email_to, $email_cc);
     $handles = id(new PhabricatorObjectHandleData($phids))->loadHandles();
 
+    // NOTE: Always set $is_new to false, because the "first" mail in the
+    // thread is the Herald notification of the commit.
+    $is_new = false;
+
     $template = id(new PhabricatorMetaMTAMail())
       ->setSubject($subject)
+      ->setVarySubject($subject)
       ->setFrom($comment->getActorPHID())
       ->setThreadID($thread_id, $is_new)
       ->addHeader('Thread-Topic', $thread_topic)
@@ -332,7 +337,7 @@ final class PhabricatorAuditCommentEditor {
 
   public static function getMailThreading($phid) {
     return array(
-      '<diffusion-audit-'.$phid.'>',
+      'diffusion-audit-'.$phid,
       'Diffusion Audit '.$phid,
     );
   }
diff --git a/src/applications/differential/mail/base/DifferentialMail.php b/src/applications/differential/mail/base/DifferentialMail.php
index 686eecbc4c..9694306ef7 100644
--- a/src/applications/differential/mail/base/DifferentialMail.php
+++ b/src/applications/differential/mail/base/DifferentialMail.php
@@ -34,7 +34,14 @@ abstract class DifferentialMail {
   protected $replyHandler;
   protected $parentMessageID;
 
-  abstract protected function renderSubject();
+  protected function renderSubject() {
+    $revision = $this->getRevision();
+    $title = $revision->getTitle();
+    $id = $revision->getID();
+    return "D{$id}: {$title}";
+  }
+
+  abstract protected function renderVarySubject();
   abstract protected function renderBody();
 
   public function setActorHandle($actor_handle) {
@@ -70,10 +77,11 @@ abstract class DifferentialMail {
       throw new Exception('No "To:" users provided!');
     }
 
-    $cc_phids    = $this->getCCPHIDs();
-    $subject     = $this->buildSubject();
-    $body        = $this->buildBody();
-    $attachments = $this->buildAttachments();
+    $cc_phids     = $this->getCCPHIDs();
+    $subject      = $this->buildSubject();
+    $vary_subject = $this->buildVarySubject();
+    $body         = $this->buildBody();
+    $attachments  = $this->buildAttachments();
 
     $template = new PhabricatorMetaMTAMail();
     $actor_handle = $this->getActorHandle();
@@ -85,6 +93,7 @@ abstract class DifferentialMail {
 
     $template
       ->setSubject($subject)
+      ->setVarySubject($vary_subject)
       ->setBody($body)
       ->setIsHTML($this->shouldMarkMailAsHTML())
       ->setParentMessageID($this->parentMessageID)
@@ -197,6 +206,10 @@ abstract class DifferentialMail {
     return trim($this->getSubjectPrefix().' '.$this->renderSubject());
   }
 
+  protected function buildVarySubject() {
+    return trim($this->getSubjectPrefix().' '.$this->renderVarySubject());
+  }
+
   protected function shouldMarkMailAsHTML() {
     return false;
   }
@@ -320,8 +333,7 @@ EOTEXT;
 
   protected function getThreadID() {
     $phid = $this->getRevision()->getPHID();
-    $domain = PhabricatorEnv::getEnvConfig('metamta.domain');
-    return "<differential-rev-{$phid}-req@{$domain}>";
+    return "differential-rev-{$phid}-req";
   }
 
   public function setComment($comment) {
diff --git a/src/applications/differential/mail/ccwelcome/DifferentialCCWelcomeMail.php b/src/applications/differential/mail/ccwelcome/DifferentialCCWelcomeMail.php
index a7db4f6ce9..85776461de 100644
--- a/src/applications/differential/mail/ccwelcome/DifferentialCCWelcomeMail.php
+++ b/src/applications/differential/mail/ccwelcome/DifferentialCCWelcomeMail.php
@@ -18,9 +18,8 @@
 
 final class DifferentialCCWelcomeMail extends DifferentialReviewRequestMail {
 
-  protected function renderSubject() {
-    $revision = $this->getRevision();
-    return 'Added to CC: '.$revision->getTitle();
+  protected function renderVarySubject() {
+    return '[Added to CC] '.$this->renderSubject();
   }
 
   protected function renderBody() {
diff --git a/src/applications/differential/mail/comment/DifferentialCommentMail.php b/src/applications/differential/mail/comment/DifferentialCommentMail.php
index 03973d33a5..fb78849b5e 100644
--- a/src/applications/differential/mail/comment/DifferentialCommentMail.php
+++ b/src/applications/differential/mail/comment/DifferentialCommentMail.php
@@ -75,13 +75,9 @@ final class DifferentialCommentMail extends DifferentialMail {
     return $tags;
   }
 
-  protected function renderSubject() {
+  protected function renderVarySubject() {
     $verb = ucwords($this->getVerb());
-    $revision = $this->getRevision();
-    $title = $revision->getTitle();
-    $id = $revision->getID();
-    $subject = "[{$verb}] D{$id}: {$title}";
-    return $subject;
+    return "[{$verb}] ".$this->renderSubject();
   }
 
   protected function getVerb() {
diff --git a/src/applications/differential/mail/diffcontent/DifferentialDiffContentMail.php b/src/applications/differential/mail/diffcontent/DifferentialDiffContentMail.php
index f021ae4d2f..110c98117c 100644
--- a/src/applications/differential/mail/diffcontent/DifferentialDiffContentMail.php
+++ b/src/applications/differential/mail/diffcontent/DifferentialDiffContentMail.php
@@ -25,8 +25,8 @@ final class DifferentialDiffContentMail extends DifferentialMail {
     $this->content = $content;
   }
 
-  protected function renderSubject() {
-    return "Content: ".$this->getRevision()->getTitle();
+  protected function renderVarySubject() {
+    return '[Content] '.$this->renderSubject();
   }
 
   protected function renderBody() {
diff --git a/src/applications/differential/mail/exception/DifferentialExceptionMail.php b/src/applications/differential/mail/exception/DifferentialExceptionMail.php
index a0484c7f52..ef11cf9e6b 100644
--- a/src/applications/differential/mail/exception/DifferentialExceptionMail.php
+++ b/src/applications/differential/mail/exception/DifferentialExceptionMail.php
@@ -36,6 +36,10 @@ final class DifferentialExceptionMail extends DifferentialMail {
     return "Exception: unable to process your mail request.";
   }
 
+  protected function renderVarySubject() {
+    return $this->renderSubject();
+  }
+
   protected function buildBody() {
     $exception = $this->exception;
     $original_body = $this->originalBody;
diff --git a/src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php b/src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
index b1d1b46182..f3167caa64 100644
--- a/src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
+++ b/src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
@@ -18,7 +18,7 @@
 
 final class DifferentialNewDiffMail extends DifferentialReviewRequestMail {
 
-  protected function renderSubject() {
+  protected function renderVarySubject() {
     $revision = $this->getRevision();
     $line_count = $revision->getLineCount();
     $lines = ($line_count == 1 ? "1 line" : "{$line_count} lines");
@@ -29,21 +29,7 @@ final class DifferentialNewDiffMail extends DifferentialReviewRequestMail {
       $verb = 'Updated';
     }
 
-    $revision_id = $revision->getID();
-    $revision_title = $revision->getTitle();
-
-    return "[{$verb}, {$lines}] D{$revision_id}: {$revision_title}";
-  }
-
-  protected function buildSubject() {
-    if (!$this->isFirstMailToRecipients()) {
-      return parent::buildSubject();
-    }
-
-    $prefix = $this->getSubjectPrefix();
-    $subject = $this->renderSubject();
-
-    return trim("{$prefix} {$subject}");
+    return "[{$verb}, {$lines}] ".$this->renderSubject();
   }
 
   protected function renderBody() {
diff --git a/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php b/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
index ac22b5ee54..94dfa2eee3 100644
--- a/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
+++ b/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
@@ -266,16 +266,18 @@ final class ManiphestTransactionEditor {
         "  ".$reply_instructions."\n";
     }
 
-    $thread_id = '<maniphest-task-'.$task->getPHID().'>';
+    $thread_id = 'maniphest-task-'.$task->getPHID();
     $task_id = $task->getID();
     $title = $task->getTitle();
     $prefix = $this->getSubjectPrefix();
-    $subject = trim("{$prefix} [{$action}] T{$task_id}: {$title}");
+    $subject = trim("{$prefix} T{$task_id}: {$title}");
+    $vary_subject = trim("{$prefix} [{$action}] T{$task_id}: {$title}");
 
     $mailtags = $this->getMailTags($transactions);
 
     $template = id(new PhabricatorMetaMTAMail())
       ->setSubject($subject)
+      ->setVarySubject($vary_subject)
       ->setFrom($transaction->getAuthorPHID())
       ->setParentMessageID($this->parentMessageID)
       ->addHeader('Thread-Topic', 'Maniphest Task '.$task->getID())
diff --git a/src/applications/metamta/replyhandler/base/PhabricatorMailReplyHandler.php b/src/applications/metamta/replyhandler/base/PhabricatorMailReplyHandler.php
index c4d7afa79b..57ffee38dc 100644
--- a/src/applications/metamta/replyhandler/base/PhabricatorMailReplyHandler.php
+++ b/src/applications/metamta/replyhandler/base/PhabricatorMailReplyHandler.php
@@ -80,22 +80,26 @@ abstract class PhabricatorMailReplyHandler {
 
     $result = array();
 
-    // If private replies are not supported, simply send one email to all
-    // recipients and CCs. This covers cases where we have no reply handler,
-    // or we have a public reply handler.
-    if (!$this->supportsPrivateReplies()) {
-      $mail = clone $mail_template;
-      $mail->addTos(mpull($to_handles, 'getPHID'));
-      $mail->addCCs(mpull($cc_handles, 'getPHID'));
+    // If MetaMTA is configured to always multiplex, skip the single-email
+    // case.
+    if (!PhabricatorMetaMTAMail::shouldMultiplexAllMail()) {
+      // If private replies are not supported, simply send one email to all
+      // recipients and CCs. This covers cases where we have no reply handler,
+      // or we have a public reply handler.
+      if (!$this->supportsPrivateReplies()) {
+        $mail = clone $mail_template;
+        $mail->addTos(mpull($to_handles, 'getPHID'));
+        $mail->addCCs(mpull($cc_handles, 'getPHID'));
 
-      if ($this->supportsPublicReplies()) {
-        $reply_to = $this->getPublicReplyHandlerEmailAddress();
-        $mail->setReplyTo($reply_to);
+        if ($this->supportsPublicReplies()) {
+          $reply_to = $this->getPublicReplyHandlerEmailAddress();
+          $mail->setReplyTo($reply_to);
+        }
+
+        $result[] = $mail;
+
+        return $result;
       }
-
-      $result[] = $mail;
-
-      return $result;
     }
 
     // Merge all the recipients together. TODO: We could keep the CCs as real
diff --git a/src/applications/metamta/replyhandler/base/__init__.php b/src/applications/metamta/replyhandler/base/__init__.php
index 145c5f6fd3..02f6d6d33d 100644
--- a/src/applications/metamta/replyhandler/base/__init__.php
+++ b/src/applications/metamta/replyhandler/base/__init__.php
@@ -6,6 +6,7 @@
 
 
 
+phutil_require_module('phabricator', 'applications/metamta/storage/mail');
 phutil_require_module('phabricator', 'applications/metamta/storage/receivedmail');
 phutil_require_module('phabricator', 'applications/phid/constants');
 phutil_require_module('phabricator', 'infrastructure/env');
diff --git a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
index 458c7545c0..3240ad2e77 100644
--- a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
@@ -149,6 +149,11 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
     return $this;
   }
 
+  public function setVarySubject($subject) {
+    $this->setParam('vary-subject', $subject);
+    return $this;
+  }
+
   public function setBody($body) {
     $this->setParam('body', $body);
     return $this;
@@ -395,23 +400,44 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
             $mailer->setBody($value);
             break;
           case 'subject':
+            // Only try to use preferences if everything is multiplexed, so we
+            // get consistent behavior.
+            $use_prefs = self::shouldMultiplexAllMail();
+
+            $prefs = null;
+            if ($use_prefs) {
+              $to = idx($params, 'to', array());
+              $user = id(new PhabricatorUser())->loadOneWhere(
+                'phid = %s',
+                head($to));
+              if ($user) {
+                $prefs = $user->loadPreferences();
+              }
+            }
+
+            $alt_subject = idx($params, 'vary-subject');
+            if ($alt_subject) {
+              $use_subject = PhabricatorEnv::getEnvConfig(
+                'metamta.vary-subjects');
+
+              if ($prefs) {
+                $use_subject = $prefs->getPreference(
+                  PhabricatorUserPreferences::PREFERENCE_VARY_SUBJECT,
+                  $use_subject);
+              }
+
+              if ($use_subject) {
+                $value = $alt_subject;
+              }
+            }
+
             if ($is_threaded) {
               $add_re = PhabricatorEnv::getEnvConfig('metamta.re-prefix');
 
-              // If this message has a single recipient, respect their "Re:"
-              // preference. Otherwise, use the global setting.
-
-              $to = idx($params, 'to', array());
-              $cc = idx($params, 'cc', array());
-              if (count($to) == 1 && count($cc) == 0) {
-                $user = id(new PhabricatorUser())->loadOneWhere(
-                  'phid = %s',
-                  head($to));
-                if ($user) {
-                  $prefs = $user->loadPreferences();
-                  $pref_key = PhabricatorUserPreferences::PREFERENCE_RE_PREFIX;
-                  $add_re = $prefs->getPreference($pref_key, $add_re);
-                }
+              if ($prefs) {
+                $add_re = $prefs->getPreference(
+                  PhabricatorUserPreferences::PREFERENCE_RE_PREFIX,
+                  $add_re);
               }
 
               if ($add_re) {
@@ -434,6 +460,14 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
             }
             break;
           case 'thread-id':
+
+            // NOTE: Gmail freaks out about In-Reply-To and References which
+            // aren't in the form "<string@domain.tld>"; this is also required
+            // by RFC 2822, although some clients are more liberal in what they
+            // accept.
+            $domain = PhabricatorEnv::getEnvConfig('metamta.domain');
+            $value = '<'.$value.'@'.$domain.'>';
+
             if ($is_first && $mailer->supportsMessageIDHeader()) {
               $mailer->addHeader('Message-ID',  $value);
             } else {
@@ -456,6 +490,9 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
           case 'mailtags':
             // Handled below.
             break;
+          case 'vary-subject':
+            // Handled above.
+            break;
           default:
             // Just discard.
         }
@@ -634,4 +671,8 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
     return $emails;
   }
 
+  public static function shouldMultiplexAllMail() {
+    return PhabricatorEnv::getEnvConfig('metamta.one-mail-per-recipient');
+  }
+
 }
diff --git a/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php b/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
index 8b7e5c73d2..d8d3b73dc7 100644
--- a/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
+++ b/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
@@ -26,24 +26,34 @@ final class PhabricatorUserEmailPreferenceSettingsPanelController
     $preferences = $user->loadPreferences();
 
     $pref_re_prefix = PhabricatorUserPreferences::PREFERENCE_RE_PREFIX;
+    $pref_vary = PhabricatorUserPreferences::PREFERENCE_VARY_SUBJECT;
     $pref_no_self_mail = PhabricatorUserPreferences::PREFERENCE_NO_SELF_MAIL;
 
     $errors = array();
     if ($request->isFormPost()) {
 
-      if ($request->getStr($pref_re_prefix) == 'default') {
-        $preferences->unsetPreference($pref_re_prefix);
-      } else {
-        $preferences->setPreference(
-          $pref_re_prefix,
-          $request->getBool($pref_re_prefix));
+      if (PhabricatorMetaMTAMail::shouldMultiplexAllMail()) {
+        if ($request->getStr($pref_re_prefix) == 'default') {
+          $preferences->unsetPreference($pref_re_prefix);
+        } else {
+          $preferences->setPreference(
+            $pref_re_prefix,
+            $request->getBool($pref_re_prefix));
+        }
+
+        if ($request->getStr($pref_vary) == 'default') {
+          $preferences->unsetPreference($pref_vary);
+        } else {
+          $preferences->setPreference(
+            $pref_vary,
+            $request->getBool($pref_vary));
+        }
       }
 
       $preferences->setPreference(
         $pref_no_self_mail,
         $request->getStr($pref_no_self_mail));
 
-
       $new_tags = $request->getArr('mailtags');
       $mailtags = $preferences->getPreference('mailtags', array());
       foreach ($this->getMailTags() as $key => $label) {
@@ -75,15 +85,28 @@ final class PhabricatorUserEmailPreferenceSettingsPanelController
       ? 'Enabled'
       : 'Disabled';
 
+    $vary_default = PhabricatorEnv::getEnvConfig('metamta.vary-subjects')
+      ? 'Vary'
+      : 'Do Not Vary';
+
     $re_prefix_value = $preferences->getPreference($pref_re_prefix);
     if ($re_prefix_value === null) {
-      $re_prefix_value = 'defualt';
+      $re_prefix_value = 'default';
     } else {
       $re_prefix_value = $re_prefix_value
         ? 'true'
         : 'false';
     }
 
+    $vary_value = $preferences->getPreference($pref_vary);
+    if ($vary_value === null) {
+      $vary_value = 'default';
+    } else {
+      $vary_value = $vary_value
+        ? 'true'
+        : 'false';
+    }
+
     $form = new AphrontFormView();
     $form
       ->setUser($user)
@@ -97,21 +120,39 @@ final class PhabricatorUserEmailPreferenceSettingsPanelController
               '1' => 'Do not send me an email when I take an action',
             ))
           ->setCaption('You can disable email about your own actions.')
-          ->setValue($preferences->getPreference($pref_no_self_mail, 0)))
-      ->appendChild(
-        id(new AphrontFormSelectControl())
-          ->setLabel('Add "Re:" Prefix')
-          ->setName($pref_re_prefix)
-          ->setCaption(
-            'Enable this option to fix threading in Mail.app on OS X Lion, '.
-            'or if you like "Re:" in your email subjects.')
-          ->setOptions(
-            array(
-              'default'   => 'Use Server Default ('.$re_prefix_default.')',
-              'true'      => 'Enable "Re:" prefix',
-              'false'     => 'Disable "Re:" prefix',
-            ))
-          ->setValue($re_prefix_value));
+          ->setValue($preferences->getPreference($pref_no_self_mail, 0)));
+
+    if (PhabricatorMetaMTAMail::shouldMultiplexAllMail()) {
+      $form
+        ->appendChild(
+          id(new AphrontFormSelectControl())
+            ->setLabel('Add "Re:" Prefix')
+            ->setName($pref_re_prefix)
+            ->setCaption(
+              'Enable this option to fix threading in Mail.app on OS X Lion, '.
+              'or if you like "Re:" in your email subjects.')
+            ->setOptions(
+              array(
+                'default'   => 'Use Server Default ('.$re_prefix_default.')',
+                'true'      => 'Enable "Re:" prefix',
+                'false'     => 'Disable "Re:" prefix',
+              ))
+            ->setValue($re_prefix_value))
+        ->appendChild(
+          id(new AphrontFormSelectControl())
+            ->setLabel('Vary Subjects')
+            ->setName($pref_vary)
+            ->setCaption(
+              'This option adds more information email subjects, but may '.
+              'break threading in some clients.')
+            ->setOptions(
+              array(
+                'default'   => 'Use Server Default ('.$vary_default.')',
+                'true'      => 'Vary Subjects',
+                'false'     => 'Do Not Vary Subjects',
+              ))
+            ->setValue($vary_value));
+    }
 
     $form
       ->appendChild(
diff --git a/src/applications/people/controller/settings/panels/emailpref/__init__.php b/src/applications/people/controller/settings/panels/emailpref/__init__.php
index c6bf80fd00..21869c2bd2 100644
--- a/src/applications/people/controller/settings/panels/emailpref/__init__.php
+++ b/src/applications/people/controller/settings/panels/emailpref/__init__.php
@@ -8,6 +8,7 @@
 
 phutil_require_module('phabricator', 'aphront/response/redirect');
 phutil_require_module('phabricator', 'applications/metamta/constants/notificationtype');
+phutil_require_module('phabricator', 'applications/metamta/storage/mail');
 phutil_require_module('phabricator', 'applications/people/controller/settings/panels/base');
 phutil_require_module('phabricator', 'applications/people/storage/preferences');
 phutil_require_module('phabricator', 'infrastructure/env');
diff --git a/src/applications/people/storage/preferences/PhabricatorUserPreferences.php b/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
index 0e8339b96c..c1512a0a00 100644
--- a/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
+++ b/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
@@ -25,6 +25,7 @@ final class PhabricatorUserPreferences extends PhabricatorUserDAO {
   const PREFERENCE_RE_PREFIX       = 're-prefix';
   const PREFERENCE_NO_SELF_MAIL    = 'self-mail';
   const PREFERENCE_MAILTAGS        = 'mailtags';
+  const PREFERENCE_VARY_SUBJECT    = 'vary-subject';
 
   const PREFERENCE_SEARCHBAR_JUMP  = 'searchbar-jump';
   const PREFERENCE_SEARCH_SHORTCUT = 'search-shortcut';
diff --git a/src/applications/repository/worker/herald/PhabricatorRepositoryCommitHeraldWorker.php b/src/applications/repository/worker/herald/PhabricatorRepositoryCommitHeraldWorker.php
index ad35e89df2..203dee9e33 100644
--- a/src/applications/repository/worker/herald/PhabricatorRepositoryCommitHeraldWorker.php
+++ b/src/applications/repository/worker/herald/PhabricatorRepositoryCommitHeraldWorker.php
@@ -150,7 +150,10 @@ WHY DID I GET THIS EMAIL?
 
 EOBODY;
 
-    $subject = "[Herald/Commit] {$commit_name} ({$who}) {$name}";
+    $prefix = PhabricatorEnv::getEnvConfig('metamta.diffusion.subject-prefix');
+
+    $subject = trim("{$prefix} {$commit_name}: {$name}");
+    $vary_subject = trim("{$prefix} [Commit] {$commit_name}: {$name}");
 
     $threading = PhabricatorAuditCommentEditor::getMailThreading(
       $commit->getPHID());
@@ -159,6 +162,7 @@ EOBODY;
     $template = new PhabricatorMetaMTAMail();
     $template->setRelatedPHID($commit->getPHID());
     $template->setSubject($subject);
+    $template->setVarySubject($subject);
     $template->setBody($body);
     $template->setThreadID($thread_id, $is_new = true);
     $template->addHeader('Thread-Topic', $thread_topic);