使用 Java Socket 构建邮件客户端：解决认证问题

import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;
import java.io.*;
import java.util.Base64;

class MailSender {

    public static void main(String[] args) {
        try {
            String smtpServer = "smtp.example.com"; // 替换为你的 SMTP 服务器地址
            int smtpPort = 465; // 替换为你的 SMTP 服务器端口
            String username = "your_username"; // 替换为你的用户名
            String password = "your_password"; // 替换为你的密码
            String sender = "sender@example.com"; // 替换为你的发件人邮箱地址
            String recipient = "recipient@example.com"; // 替换为你的收件人邮箱地址
            String subject = "Test Email";
            String body = "This is a test email sent using Java Socket.";

            SSLSocketFactory sslSocketFactory = (SSLSocketFactory) SSLSocketFactory.getDefault();
            SSLSocket socket = (SSLSocket) sslSocketFactory.createSocket(smtpServer, smtpPort);

            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

            // 1. 连接
            System.out.println("S: " + in.readLine());

            // 2. EHLO
            out.println("EHLO example.com");
            System.out.println("C: EHLO example.com");
            String response;
            while ((response = in.readLine()) != null) {
                System.out.println("S: " + response);
                if (response.startsWith("250 ")) {
                    break;
                }
            }

            // 3. AUTH PLAIN
            String authString = "\0" + username + "\0" + password;
            String encodedAuthString = Base64.getEncoder().encodeToString(authString.getBytes());
            out.println("AUTH PLAIN " + encodedAuthString);
            System.out.println("C: AUTH PLAIN " + encodedAuthString);
            System.out.println("S: " + in.readLine());

            // 4. MAIL FROM
            out.println("MAIL FROM: <" + sender + ">");
            System.out.println("C: MAIL FROM: <" + sender + ">");
            System.out.println("S: " + in.readLine());

            // 5. RCPT TO
            out.println("RCPT TO: <" + recipient + ">");
            System.out.println("C: RCPT TO: <" + recipient + ">");
            System.out.println("S: " + in.readLine());

            // 6. DATA
            out.println("DATA");
            System.out.println("C: DATA");
            System.out.println("S: " + in.readLine());

            // 7. Email Content
            out.println("Subject: " + subject);
            out.println(body);
            out.println(".");
            System.out.println("C: [Email Content]");
            System.out.println("S: " + in.readLine());

            // 8. QUIT
            out.println("QUIT");
            System.out.println("C: QUIT");
            System.out.println("S: " + in.readLine());

            out.close();
            in.close();
            socket.close();

            System.out.println("Email sent successfully!");

        } catch (Exception e) {
            System.err.println("Error sending email: " + e.getMessage());
            e.printStackTrace();
        }
    }
}