diff --git a/src/Symfony/Component/Mailer/Bridge/Postal/Tests/Transport/PostalApiTransportTest.php b/src/Symfony/Component/Mailer/Bridge/Postal/Tests/Transport/PostalApiTransportTest.php
index b064380ad086a..1938ffde6c69e 100644
--- a/src/Symfony/Component/Mailer/Bridge/Postal/Tests/Transport/PostalApiTransportTest.php
+++ b/src/Symfony/Component/Mailer/Bridge/Postal/Tests/Transport/PostalApiTransportTest.php
@@ -64,7 +64,7 @@ public function testSend()
             $this->assertSame(base64_encode('some attachment'), $body['attachments'][0]['data']);
             $this->assertSame('foo@bar.fr', $body['reply_to']);
 
-            return new JsonMockResponse(['message_id' => 'foobar'], [
+            return new JsonMockResponse(['data' => ['message_id' => 'foobar']], [
                 'http_code' => 200,
             ]);
         });
diff --git a/src/Symfony/Component/Mailer/Bridge/Postal/Transport/PostalApiTransport.php b/src/Symfony/Component/Mailer/Bridge/Postal/Transport/PostalApiTransport.php
index 3593f2e78c91b..028d135bd5132 100644
--- a/src/Symfony/Component/Mailer/Bridge/Postal/Transport/PostalApiTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/Postal/Transport/PostalApiTransport.php
@@ -64,7 +64,7 @@ protected function doSendApi(SentMessage $sentMessage, Email $email, Envelope $e
             throw new HttpTransportException('Unable to send an email: '.$result['message'].\sprintf(' (code %d).', $statusCode), $response);
         }
 
-        $sentMessage->setMessageId($result['message_id']);
+        $sentMessage->setMessageId($result['data']['message_id']);
 
         return $response;
     }