Mailgun\Messages\MessageBuilder::addCustomHeader PHP Method

    public function addCustomHeader($headerName, $headerData)
    {
        if (!preg_match('/^h:/i', $headerName)) {
            $headerName = 'h:' . $headerName;
        }
        $this->message[$headerName] = [$headerData];
        return $this->message[$headerName];
    }

 /**
  * Append a custom MIME header to a message.
  *
  * @param string $key
  * @param string $value
  *
  * @return \Bogardo\Mailgun\Mail\Message
  */
 public function header($key, $value)
 {
     $this->messageBuilder->addCustomHeader($key, $value);
     return $this;
 }