Java裸Socket实现SMTP邮件发送：详解认证机制与EHLO协议应用

echo -ne 'your_usernameyour_password' | openssl base64

import javax.net.ssl.SSLSocket; import javax.net.ssl.SSLSocketFactory; import java.io.*; import java.util.Base64; // 用于Base64编码  public class mailClientWithAuth {      // 邮件发送者和接收者     private static final String FROM_EMAIL = "sender@example.com"; // 替换为你的发件邮箱     private static final String TO_EMAIL = "receiver@example.com"; // 替换为你的收件邮箱     // SMTP服务器地址和端口     private static final String SMTP_HOST = "smtp.naver.com"; // 或 smtp.gmail.com, smtp.qq.com 等     private static final int SMTP_PORT = 465; // SMTPS默认端口      // 邮箱认证凭据     private static final String USERNAME = "your_email_username"; // 替换为你的邮箱账号     private static final String PASSWORD = "your_email_password"; // 替换为你的邮箱密码或授权码      public static void sendMail() throws Exception {         String reply;          // 1. 初始化SSL Socket连接         SSLSocketFactory sslsocketf = (SSLSocketFactory) SSLSocketFactory.getDefault();         System.out.println("Connect Start to " + SMTP_HOST + ":" + SMTP_PORT);         SSLSocket socket = (SSLSocket) sslsocketf.createSocket(SMTP_HOST, SMTP_PORT);         System.out.println("Connect Success");          BufferedReader inFromServer = new BufferedReader(new InputStreamReader(socket.getInputStream()));         PrintWriter outToServer = new PrintWriter(socket.getOutputStream(), true);          // 2. 接收服务器初始欢迎消息         reply = inFromServer.readLine();         if (reply == null || !reply.startsWith("220")) {             System.out.println("Server Connect Fail: " + reply);             System.exit(0);         }         System.out.println("Server Connect Success! " + reply);          // 3. 发送 EHLO 命令并检查服务器能力         outToServer.println("EHLO " + SMTP_HOST.split(".")[0] + ".com"); // 使用域名的一部分作为EHLO参数         reply = inFromServer.readLine();         System.out.println(reply);         // 检查EHLO响应中是否包含AUTH能力         if (!reply.startsWith("250")) {             throw new Exception("EHLO command failed: " + reply);         }         // 读取所有EHLO响应行，直到遇到以250开头的行且后面没有破折号         while (reply.startsWith("250-")) {             reply = inFromServer.readLine();             System.out.println(reply);         }         // 假设服务器支持AUTH PLAIN，这里简化处理，实际应解析响应         // if (!reply.contains("AUTH PLAIN")) {         //     throw new Exception("Server does not support AUTH PLAIN: " + reply);         // }           // 4. 进行身份验证 (AUTH PLAIN)         // 构造认证字符串: usernamepassword 并进行Base64编码         String authString = "" + USERNAME + "" + PASSWORD;         String encodedAuthString = Base64.getEncoder().encodeToString(authString.getBytes());          outToServer.println("AUTH PLAIN " + encodedAuthString);         reply = inFromServer.readLine();         replyCheck(reply, "235"); // 235 2.0.0 Authentication successful         System.out.println("Authentication successful.");          // 5. 发送 MAIL FROM 命令         outToServer.println("MAIL FROM: <" + FROM_EMAIL + ">");         reply = inFromServer.readLine();         replyCheck(reply, "250");          // 6. 发送 RCPT TO 命令         outToServer.println("RCPT TO: <" + TO_EMAIL + ">");         reply = inFromServer.readLine();         replyCheck(reply, "250");          // 7. 发送 DATA 命令，准备发送邮件内容         outToServer.println("DATA");         reply = inFromServer.readLine();         replyCheck(reply, "354"); // 354 Start mail input; end with <CRLF>.<CRLF>          // 8. 发送邮件内容         outToServer.println("Subject: Test Mail from Java Socket Client");         outToServer.println("From: " + FROM_EMAIL);         outToServer.println("To: " + TO_EMAIL);         outToServer.println(""); // 空行分隔邮件头和邮件体         outToServer.println("Hello, this is a test email sent from a custom Java socket mail client.");         outToServer.println("This demonstrates SMTP EHLO and AUTH PLAIN authentication.");         outToServer.println("."); // 邮件内容结束标志          reply = inFromServer.readLine();         replyCheck(reply, "250"); // 250 OK, message accepted for delivery          // 9. 发送 QUIT 命令，结束会话         outToServer.println("QUIT");         reply = inFromServer.readLine();         replyCheck(reply, "221"); // 221 Bye          // 10. 关闭资源         outToServer.close();         inFromServer.close();         socket.close();     }      /**      * 检查服务器响应是否以预期的状态码开头      * @param reply 服务器响应字符串      * @param expectedStatus 预期的状态码前缀      * @throws Exception 如果响应不匹配预期状态码      */     public static void replyCheck(String reply, String expectedStatus) throws Exception {         System.out.println("Server Reply: " + reply);         if (reply == null || !reply.startsWith(expectedStatus)) {             throw new Exception("Unexpected server reply: " + reply);         }     }      public static void main(String[] args) {         try {             MailClientWithAuth.sendMail();             System.out.println("Send Mail Success!");         } catch (Exception e) {             System.err.println("Send Mail Fail");             System.err.println("Error: " + e.getMessage());             e.printStackTrace();         }     } }