javaMail 设置邮件回执

要设置邮件回执，可以使用 JavaMail API 中的 Receipt类，具体步骤如下：

1. 创建一个邮件会话对象，例如：

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

2. 创建一个 MimeMessage 对象，设置发送人、收件人、主题、内容等信息。

3. 创建一个 Receipt 对象，设置邮件回执的收件人地址：

Receipt receipt = new Receipt(new InternetAddress("receipt@example.com"));

4. 将 Receipt 对象添加到 MimeMessage 中：

message.setHeader("Disposition-Notification-To", receipt.getNotificationTo());

5. 发送邮件即可，当收件人打开邮件时，会自动发送回执。

完整代码示例：

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

public class MailSender {
    public static void main(String[] args) throws MessagingException {
        String username = "your_username";
        String password = "your_password";
        String from = "from@example.com";
        String to = "to@example.com";
        String subject = "Test Email";
        String content = "This is a test email.";

        Properties props = new Properties();
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.host", "smtp.example.com");
        props.put("mail.smtp.port", "587");

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

        MimeMessage message = new MimeMessage(session);
        message.setFrom(new InternetAddress(from));
        message.setRecipient(Message.RecipientType.TO, new InternetAddress(to));
        message.setSubject(subject);
        message.setText(content);

        Receipt receipt = new Receipt(new InternetAddress("receipt@example.com"));
        message.setHeader("Disposition-Notification-To", receipt.getNotificationTo());

        Transport.send(message);
    }
}