SendEmail.Java

 public static ClientResponse SendEmailGun(String to) {
        WebResource webresource = null;
        MultivaluedMapImpl formData = new MultivaluedMapImpl();
        try {
            Client client = Client.create();
            client.addFilter(new HTTPBasicAuthFilter("api", MAILGUN_API_KEY));
            webresource = client.resource(MAILGUN_HOST + "/messages");
            formData.add("from", "Excited User <[email protected]> ");
            formData.add("to", to);
            formData.add("subject", "Struk Belanja Anda");
            formData.add("text", "Testing PIN ADMOB");

        } catch (Exception e) {
            e.printStackTrace();
        }
        return webresource.type(MediaType.APPLICATION_FORM_URLENCODED).post(ClientResponse.class, formData);
    }