Store the SparkPost transmission ID in the header X-SparkPost-Transmi…

…ssion-ID after the message has been sent

src/Illuminate/Mail/Transport/SparkPostTransport.php

@@ -54,7 +54,7 @@ public function send(Swift_Mime_Message $message, &$failedRecipients = null)
 
         $message->setBcc([]);
 
-        $this->client->post('https://api.sparkpost.com/api/v1/transmissions', [
+        $response = $this->client->post('https://api.sparkpost.com/api/v1/transmissions', [
             'headers' => [
                 'Authorization' => $this->key,
             ],
@@ -66,6 +66,8 @@ public function send(Swift_Mime_Message $message, &$failedRecipients = null)
             ], $this->options),
         ]);
 
+        $message->getHeaders()->addTextHeader('X-SparkPost-Transmission-ID', $this->getTransmissionId($response));
+
         $this->sendPerformed($message);
 
         return $this->numberOfRecipients($message);
@@ -98,6 +100,19 @@ protected function getRecipients(Swift_Mime_Message $message)
         return $recipients;
     }
 
+    /**
+     * Get the transmission ID from the response.
+     *
+     * @param \GuzzleHttp\Psr7\Response $response
+     * @return string
+     */
+    protected function getTransmissionId($response)
+    {
+        $result = json_decode($response->getBody()->getContents());
+
+        return object_get($result, 'results.id');
+    }
+
     /**
      * Get the API key being used by the transport.
      *