Servlet 发送电子邮件

流程

Text

你的应用  --SMTP-->  邮件服务器  --SMTP-->  收件人服务器  -->  收件箱
         (认证、投递)              (DNS MX 查询)

Your app does not deliver to the recipient; it hands the message to its own SMTP server, which handles queueing, retries and bounces.

纯文本邮件

Java

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

public class Mailer {

    public static void send(String to, String subject, String body) throws Exception {
        Properties p = new Properties();
        p.put("mail.smtp.host", "smtp.example.com");
        p.put("mail.smtp.port", "587");
        p.put("mail.smtp.auth", "true");
        p.put("mail.smtp.starttls.enable", "true");

        Session session = Session.getInstance(p, new Authenticator() {
            @Override
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(
                        System.getenv("MAIL_USER"),
                        System.getenv("MAIL_PASS"));
            }
        });

        Message msg = new MimeMessage(session);
        msg.setFrom(new InternetAddress(System.getenv("MAIL_USER")));
        msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
        msg.setSubject(subject, "UTF-8");
        msg.setText(body, "UTF-8");

        Transport.send(msg);
    }
}

HTML 邮件与附件

Java

MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress(from));
msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
msg.setSubject("月度报告", "UTF-8");

Multipart multipart = new MimeMultipart("mixed");

// 正文：纯文本 + HTML 两段，邮件客户端自己挑
MimeBodyPart text = new MimeBodyPart();
text.setText("您的客户端不支持 HTML。", "UTF-8");

MimeBodyPart html = new MimeBodyPart();
html.setContent("<h1>月度报告</h1><p>详见附件。</p>", "text/html;charset=UTF-8");

MimeMultipart alt = new MimeMultipart("alternative");
alt.addBodyPart(text);
alt.addBodyPart(html);

MimeBodyPart bodyPart = new MimeBodyPart();
bodyPart.setContent(alt);
multipart.addBodyPart(bodyPart);

// 附件
MimeBodyPart file = new MimeBodyPart();
file.attachFile("/data/report.pdf");
file.setFileName(MimeUtility.encodeText("月度报告.pdf", "UTF-8", "B"));
multipart.addBodyPart(file);

msg.setContent(multipart);
Transport.send(msg);

multipart/alternative means "same content, different formats" and the client picks one; multipart/mixed means "several parallel parts" for attachments. Nesting them the wrong way shows two bodies.

常见失败原因

Symptom	Cause
535 auth failed	用了登录密码而非授权码；或未开 SMTP
Connection timed out	端口被封（25 常被云厂商禁用）
550 relay denied	发件人域名与认证账号不一致
Subject garbled	未指定 UTF-8 编码
Lands in spam	SPF/DKIM/DMARC 未配置

Sending synchronously on the request thread lets one slow SMTP server stall the whole request. In production, enqueue and send asynchronously with retries; email must not affect response time.

流程

Text

你的应用  --SMTP-->  邮件服务器  --SMTP-->  收件人服务器  -->  收件箱
         (认证、投递)              (DNS MX 查询)

应用不直接投递到收件人，只负责把邮件交给自己的 SMTP 服务器。之后的排队、重试、退信都由邮件服务器处理。

纯文本邮件

Java

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

public class Mailer {

    public static void send(String to, String subject, String body) throws Exception {
        Properties p = new Properties();
        p.put("mail.smtp.host", "smtp.example.com");
        p.put("mail.smtp.port", "587");
        p.put("mail.smtp.auth", "true");
        p.put("mail.smtp.starttls.enable", "true");

        Session session = Session.getInstance(p, new Authenticator() {
            @Override
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(
                        System.getenv("MAIL_USER"),
                        System.getenv("MAIL_PASS"));
            }
        });

        Message msg = new MimeMessage(session);
        msg.setFrom(new InternetAddress(System.getenv("MAIL_USER")));
        msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
        msg.setSubject(subject, "UTF-8");
        msg.setText(body, "UTF-8");

        Transport.send(msg);
    }
}

HTML 邮件与附件

Java

MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress(from));
msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
msg.setSubject("月度报告", "UTF-8");

Multipart multipart = new MimeMultipart("mixed");

// 正文：纯文本 + HTML 两段，邮件客户端自己挑
MimeBodyPart text = new MimeBodyPart();
text.setText("您的客户端不支持 HTML。", "UTF-8");

MimeBodyPart html = new MimeBodyPart();
html.setContent("<h1>月度报告</h1><p>详见附件。</p>", "text/html;charset=UTF-8");

MimeMultipart alt = new MimeMultipart("alternative");
alt.addBodyPart(text);
alt.addBodyPart(html);

MimeBodyPart bodyPart = new MimeBodyPart();
bodyPart.setContent(alt);
multipart.addBodyPart(bodyPart);

// 附件
MimeBodyPart file = new MimeBodyPart();
file.attachFile("/data/report.pdf");
file.setFileName(MimeUtility.encodeText("月度报告.pdf", "UTF-8", "B"));
multipart.addBodyPart(file);

msg.setContent(multipart);
Transport.send(msg);

multipart/alternative 表示「同一内容的不同格式」，客户端选最合适的显示；multipart/mixed 表示「多个并列部分」，用于附件。嵌套顺序不能反，否则邮件会显示成两个正文。

常见失败原因

现象	原因
认证失败	用了登录密码而非授权码；或未开 SMTP
连接超时	端口被封（25 常被云厂商禁用）
拒绝转发	发件人域名与认证账号不一致
主题乱码	未指定 UTF-8 编码
进垃圾箱	SPF/DKIM/DMARC 未配置

在 Servlet 的请求线程里同步发信，一个慢 SMTP 就能拖住整个请求。生产环境应该丢进队列异步发，失败重试；邮件不该影响用户操作的响应时间。

Servlet 发送电子邮件

流程

纯文本邮件

HTML 邮件与附件

常见失败原因

流程

纯文本邮件

HTML 邮件与附件

常见失败原因

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