title: Passing Unique Arguments to SendGrid Using Swiftmailer
body: <p>Hi I'm using the php mailer Swiftmailer to send through SendGrid using the SMTP API.  We can pass unique arguments to SendGrid to help track the email.  I've been successful in passing unique headers with Swiftmailer:</p>
  
  <pre><code>   $headers = $message-&gt;getHeaders();
  
     $headers-&gt;addTextHeader('AccountId', $accountId);
  </code></pre>
  
  <p>While these values are successfully injected into the header of sent email, they are not being passed back by SendGrid to my event webhook or in the control panel.  </p>
  
  <p>I also tried this, which didn't work either:</p>
  
  <pre><code>$headers = $message-&gt;getHeaders();
  
          $args = '  {
      "customerAccountNumber": "55555",
      "activationAttempt": "1",
      "New Argument 1": "New Value 1",
      "New Argument 2": "New Value 2",
      "New Argument 3": "New Value 3",
      "New Argument 4": "New Value 4"
  
  }';
          $headers-&gt;addTextHeader('unique_args', $args);
  </code></pre>
  
  <p>This is probably a question for SendGrid support but since it relates to Swiftmailer I thought I'd try here first.  Thanks!</p>