Index: conf/default.conf.php
===================================================================
--- conf/default.conf.php
+++ conf/default.conf.php
@@ -167,6 +167,10 @@
   // PhabricatorMailReplyHandler (and possibly of ManiphestReplyHandler).
   'metamta.maniphest.reply-handler' => 'ManiphestReplyHandler',
 
+  // Prefix prepended to mail sent by Maniphest. You can change this to
+  // distinguish between testing and development installs, for example.
+  'metamta.maniphest.subject-prefix' => '[Maniphest]',
+
   // See 'metamta.maniphest.reply-handler-domain'. This does the same thing,
   // but allows email replies via Differential.
   'metamta.differential.reply-handler-domain' => null,
@@ -175,6 +179,9 @@
   // affects Differential.
   'metamta.differential.reply-handler' => 'DifferentialReplyHandler',
 
+  // Prefix prepended to mail sent by Differential.
+  'metamta.differential.subject-prefix' => '[Differential]',
+
 
 // -- Auth ------------------------------------------------------------------ //
 
Index: src/applications/differential/mail/base/DifferentialMail.php
===================================================================
--- src/applications/differential/mail/base/DifferentialMail.php
+++ src/applications/differential/mail/base/DifferentialMail.php
@@ -18,8 +18,6 @@
 
 abstract class DifferentialMail {
 
-  const SUBJECT_PREFIX  = '[Differential]';
-
   protected $to = array();
   protected $cc = array();
 
@@ -115,8 +113,12 @@
     }
   }
 
+  protected function getSubjectPrefix() {
+    return PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix');
+  }
+
   protected function buildSubject() {
-    return self::SUBJECT_PREFIX.' '.$this->renderSubject();
+    return trim($this->getSubjectPrefix().' '.$this->renderSubject());
   }
 
   protected function shouldMarkMailAsHTML() {
Index: src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
===================================================================
--- src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
+++ src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php
@@ -40,11 +40,10 @@
       return parent::buildSubject();
     }
 
-    $prefix = self::SUBJECT_PREFIX;
-
+    $prefix = $this->getSubjectPrefix();
     $subject = $this->renderSubject();
 
-    return "{$prefix} {$subject}";
+    return trim("{$prefix} {$subject}");
   }
 
   protected function renderBody() {
Index: src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
===================================================================
--- src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
+++ src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
@@ -17,7 +17,6 @@
  */
 
 class ManiphestTransactionEditor {
-  const SUBJECT_PREFIX = '[Maniphest]';
 
   public function applyTransactions($task, array $transactions) {
 
@@ -130,6 +129,10 @@
     $this->sendEmail($task, $transactions, $email_to, $email_cc);
   }
 
+  protected function getSubjectPrefix() {
+    return PhabricatorEnv::getEnvConfig('metamta.maniphest.subject-prefix');
+  }
+
   private function sendEmail($task, $transactions, $email_to, $email_cc) {
     $email_to = array_filter(array_unique($email_to));
     $email_cc = array_filter(array_unique($email_cc));
@@ -193,9 +196,11 @@
     $thread_id = '<maniphest-task-'.$task->getPHID().'>';
     $task_id = $task->getID();
     $title = $task->getTitle();
+    $prefix = $this->getSubjectPrefix();
+    $subject = trim("{$prefix} [{$action}] T{$task_id}: {$title}");
 
     $template = id(new PhabricatorMetaMTAMail())
-      ->setSubject(self::SUBJECT_PREFIX." [{$action}] T{$task_id}: {$title}")
+      ->setSubject($subject)
       ->setFrom($transaction->getAuthorPHID())
       ->addHeader('Thread-Topic', 'Maniphest Task '.$task->getID())
       ->setThreadID($thread_id, $is_create)