code: public void sendAsync(final MailRequestCallback callback) {
          if (!configuration.mailSendFilter().filter(this)) return;
          prepareSend();
          request()
                  .async()
                  .post(entity(),
                        new InvocationCallback<javax.ws.rs.core.Response>() {
                            @Override
                            public void completed(javax.ws.rs.core.Response o) {
                                callback.completed(new Response(o));
                            }
  
                            @Override
                            public void failed(Throwable throwable) {
                                callback.failed(throwable);
                            }
                        });
      }
docs: Sends the email asynchronously.
  <p>
  This method returns immediately, sending the request to the Mailgun
  service in the background. It is a <strong>non-blocking</strong>
  method.
  
  @param callback the callback to be invoked upon completion or failure