diff --git a/src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php b/src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php
--- a/src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php
+++ b/src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php
@@ -68,7 +68,7 @@
   }
 
   public function supportsMessageIDHeader() {
-    return false;
+    return true;
   }
 
   public function send() {