Java Web应用程序不能在Ubuntu服务器上发送电子邮件

import javax.mail.*;
import javax.mail.internet.*;
import java.util.Properties;

public class EmailSender {
    public static void main(String[] args) {
        // 邮件服务器配置
        String host = "localhost";
        String username = "your_username";
        String password = "your_password";

        // 发件人和收件人信息
        String from = "sender@example.com";
        String to = "recipient@example.com";

        // 邮件内容
        String subject = "Hello";
        String body = "This is a test email.";

        // 配置邮件服务器属性
        Properties props = new Properties();
        props.put("mail.smtp.host", host);
        props.put("mail.smtp.auth", "true");

        // 创建会话
        Session session = Session.getInstance(props, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });

        try {
            // 创建邮件消息
            Message message = new MimeMessage(session);
            message.setFrom(new InternetAddress(from));
            message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
            message.setSubject(subject);
            message.setText(body);

            // 发送邮件
            Transport.send(message);

            System.out.println("Email sent successfully.");
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}