Merge pull request #28994 from CyrilMazur/5.8-mailgun-message-id

[5.8] Storing Mailgun Message-ID in the headers after sending

src/Illuminate/Mail/Transport/MailgunTransport.php

@@ -64,12 +64,16 @@ public function send(Swift_Mime_SimpleMessage $message, &$failedRecipients = nul
 
         $message->setBcc([]);
 
-        $this->client->request(
+        $response = $this->client->request(
             'POST',
             "https://{$this->endpoint}/v3/{$this->domain}/messages.mime",
             $this->payload($message, $to)
         );
 
+        $message->getHeaders()->addTextHeader(
+            'X-Mailgun-Message-ID', $this->getMessageId($response)
+        );
+
         $this->sendPerformed($message);
 
         return $this->numberOfRecipients($message);
@@ -129,6 +133,19 @@ protected function allContacts(Swift_Mime_SimpleMessage $message)
         );
     }
 
+    /**
+     * Get the message ID from the response.
+     *
+     * @param  \Psr\Http\Message\ResponseInterface  $response
+     * @return string
+     */
+    protected function getMessageId($response)
+    {
+        return object_get(
+            json_decode($response->getBody()->getContents()), 'id'
+        );
+    }
+
     /**
      * Get the API key being used by the transport.
      *