无法连接到SMTP主机：localhost，端口：25

Question

我正在尝试使用SMTP发送忘记密码的电子邮件。但是我得到如下的例外

javax.mail.SendFailedException: Sending failed;
  nested exception is:
    class javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 25;

以下是java代码

    String to = email;


            String from = "XXXXXXX@gmail.com";          
            String host = "localhost";
            Properties properties = System.getProperties();    
            properties.setProperty("mail.smtp.host", host);         
            Session session = Session.getDefaultInstance(properties);

            try {
                MimeMessage message = new MimeMessage(session);             
                message.setFrom(new InternetAddress(from));
                message.addRecipient(Message.RecipientType.TO, new InternetAddress(
                        to));
                message.setSubject("This is the Subject Line!");
                message.setContent("<h1>This is actual message</h1>", "text/html");
                Transport.send(message);
}

任何人都可以告诉我如何使用SMTP发送电子邮件