code: public Response send() {
          if (!configuration.mailSendFilter().filter(this)) return null;
          prepareSend();
          return new Response(request().post(entity()));
      }
docs: Sends the email.
  <p>
  This method send the request to the Mailgun service. It is a
  <strong>blocking</strong> method so it will return upon request
  completion.
  
  @return the response from the Mailgun service or null if the message
  is not sent (filtered by {@link MailSendFilter}