Index: conf/default.conf.php
===================================================================
--- conf/default.conf.php
+++ conf/default.conf.php
@@ -243,6 +243,16 @@
   // Email" in the documentation for more information.
   'metamta.maniphest.public-create-email' => null,
 
+  // If you enable 'metamta.public-replies', Phabricator uses "From" to
+  // authenticate users. You can additionally enable this setting to try to
+  // authenticate with 'Reply-To'. Note that this is completely spoofable and
+  // insecure (any user can set any 'Reply-To' address) but depending on the
+  // nature of your install or other deliverability conditions this might be
+  // okay. Generally, you can't do much more by spoofing Reply-To than be
+  // annoying (you can write but not read content). But, you know, this is
+  // still **COMPLETELY INSECURE**.
+  'metamta.insecure-auth-with-reply-to' => false,
+
 
 // -- Auth ------------------------------------------------------------------ //
 
Index: src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
===================================================================
--- src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
+++ src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
@@ -224,9 +224,26 @@
     $from = idx($this->headers, 'from');
     $from = $this->getRawEmailAddress($from);
 
-    return id(new PhabricatorUser())->loadOneWhere(
+    $user = id(new PhabricatorUser())->loadOneWhere(
       'email = %s',
       $from);
+
+    // If Phabricator is configured to allow "Reply-To" authentication, try
+    // the "Reply-To" address if we failed to match the "From" address.
+    $config_key = 'metamta.insecure-auth-with-reply-to';
+    $allow_reply_to = PhabricatorEnv::getEnvConfig($config_key);
+
+    if (!$user && $allow_reply_to) {
+      $reply_to = idx($this->headers, 'reply-to');
+      $reply_to = $this->getRawEmailAddress($reply_to);
+      if ($reply_to) {
+        $user = id(new PhabricatorUser())->loadOneWhere(
+          'email = %s',
+          $reply_to);
+      }
+    }
+
+    return $user;
   }
 
 }