依賴準備

<!-- Maven 坐標 (Jakarta Mail 2.x) -->
<dependency>
    <groupId>com.sun.mail</groupId>
    <artifactId>jakarta.mail</artifactId>
    <version>2.0.1</version>
</dependency>
<!-- 或舊版 javax.mail -->
<dependency>
    <groupId>com.sun.mail</groupId>
    <artifactId>javax.mail</artifactId>
    <version>1.6.2</version>
</dependency>

核心類：Session(連接配置)、Message(郵件對象)、Transport(發送)、Store/Folder(接收)、MimeMessage(MIME 郵件)、MimeMultipart(混合內容)、MimeBodyPart(郵件部件)。

基礎配置：Session 與認證

Properties props = new Properties();
props.put("mail.smtp.auth", "true");
props.put("mail.transport.protocol", "smtp");
props.put("mail.smtp.host", "smtp.126.com");
props.put("mail.smtp.port", "25"); // 或 465(SSL), 587(STARTTLS)

Session session = Session.getInstance(props, new Authenticator() {
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication("user@126.com", "password");
    }
});
session.setDebug(true); // 控制台打印 SMTP 交互細節

場景一：純文本郵件

MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress("sender@126.com"));
msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse("receiver@qq.com"));
msg.setSubject("測試郵件");
msg.setText("你好，這是純文本內容");
Transport.send(msg);

場景二：多收件人、抄送、密送、HTML 內容

msg.setRecipients(Message.RecipientType.TO, 
    InternetAddress.parse("to1@qq.com, to2@163.com"));
msg.setRecipients(Message.RecipientType.CC, 
    InternetAddress.parse("cc@hotmail.com")); // 抄送，收件人可見
msg.setRecipients(Message.RecipientType.BCC, 
    InternetAddress.parse("bcc@gmail.com"));  // 密送，收件人不可見

msg.setContent("
HTML 富文本
支持圖片、鏈接、樣式
", 
    "text/html;charset=utf-8");
Transport.send(msg);

場景三：複雜郵件 — 附件 + 內嵌圖片 + HTML 正文

MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress("sender@126.com", "發件人姓名", "utf-8"));
msg.setSubject("帶附件和圖片的郵件", "utf-8");
msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse("receiver@qq.com"));

// 頂層容器：mixed (附件與正文並列)
MimeMultipart topMultipart = new MimeMultipart("mixed");
msg.setContent(topMultipart);

// --- 附件 1 ---
MimeBodyPart attach1 = new MimeBodyPart();
attach1.attachFile(new File("config.txt")); // JDK 1.6+ 便捷方法
attach1.setFileName(MimeUtility.encodeText("配置文件.txt")); // 中文文件名編碼
topMultipart.addBodyPart(attach1);

// --- 附件 2 ---
MimeBodyPart attach2 = new MimeBodyPart();
attach2.attachFile(new File("photo.jpg"));
attach2.setFileName(MimeUtility.encodeText("照片.jpg"));
topMultipart.addBodyPart(attach2);

// --- 正文區域：related (HTML + 內嵌圖片) ---
MimeMultipart bodyMultipart = new MimeMultipart("related");
MimeBodyPart htmlPart = new MimeBodyPart();
MimeBodyPart imagePart = new MimeBodyPart();

htmlPart.setContent(
    "
報告正文
", // cid 引用內嵌圖片
    "text/html;charset=utf-8"
);
imagePart.attachFile(new File("chart.png"));
imagePart.setContentID(""); // 匹配 cid:chart.png
imagePart.setDisposition(MimeBodyPart.INLINE);

bodyMultipart.addBodyPart(htmlPart);
bodyMultipart.addBodyPart(imagePart);

// 將正文區域加入頂層
MimeBodyPart bodyWrapper = new MimeBodyPart();
bodyWrapper.setContent(bodyMultipart);
topMultipart.addBodyPart(bodyWrapper);

msg.saveChanges(); // 重要：生成 Message-ID 等頭部
Transport.send(msg);

接收郵件：POP3 / IMAP

Properties props = new Properties();
props.put("mail.pop3.ssl.enable", "true");
props.put("mail.pop3.host", "pop.163.com");
props.put("mail.pop3.port", "995");

Session session = Session.getDefaultInstance(props);
Store store = session.getStore("pop3");
store.connect("user@163.com", "password");

Folder inbox = store.getFolder("INBOX");
inbox.open(Folder.READ_ONLY);

Message[] messages = inbox.getMessages();
// 獲取最新一封
Message latest = messages[messages.length - 1];
System.out.println("發件人: " + latest.getFrom()[0]);
System.out.println("主題: " + latest.getSubject());
System.out.println("時間: " + latest.getSentDate());

// 解析內容 (遞歸處理 Multipart)
Object content = latest.getContent();
if (content instanceof Multipart) {
    parseMultipart((Multipart) content);
}

inbox.close(false);
store.close();

常見問題與解決方案

最佳實踐清單

• ✅ 生產環境必須用 SSL/TLS (465/587/993/995 端口)
• ✅ 密碼/授權碼配置中心管理，絕不硬編碼
• ✅ 發送異常重試機制 (指數退避) + 死信隊列
• ✅ 批量發送異步化 (消息隊列 + 消費者限流)
• ✅ 郵件模板外部化 (Thymeleaf/FreeMarker)，內容與代碼分離
• ✅ 關鍵日志：Message-ID、收件人、發送耗時、結果碼