使用Socket类基于SMTP协议发送邮件的Java代码故障排查

我正在开发一个基于SMTP协议发送邮件的Java类，不使用任何简化邮件发送的库（如MimeMessage或MailUtility）。以下是目前完成的代码：

public class MailSender {

    protected String localMachine;
    protected String localSMTPServer;
    private String textMail;

    private String mailFrom;

    public MailSender(String localSMTPServer, String mailFrom) {
        try {
            this.textMail = "null";
            this.localSMTPServer = localSMTPServer;
            this.mailFrom = mailFrom;

            InetAddress localIP = InetAddress.getLocalHost();
            this.localMachine = localIP.getHostAddress();

        } catch (UnknownHostException ex) {
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    //getter and setter
    public void smtp(String command) {
        try {
            System.out.println(sIn.readLine());
            os.write((command + "\n").getBytes());
            System.out.println("end");
        } catch (IOException ex) {
            System.out.println(ex);
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }           
    }

    BufferedReader sIn;
    OutputStream os;

    public Boolean trySend(String destUser, String subject, String message) {
        Socket socket;
        try {
            socket = new Socket(localSMTPServer, 587);

            OutputStreamWriter sOutW = new OutputStreamWriter(socket.getOutputStream(), "ASCII");

            PrintWriter sOut = new PrintWriter(sOutW, true);
            InputStreamReader sInR = new InputStreamReader(socket.getInputStream(), "ASCII");
            
            sIn = new BufferedReader(sInR);
            os = socket.getOutputStream();
            this.nextSMTPCode(sIn, 220);
            sOut.println("EHLO");
            this.nextSMTPCode(sIn, 250);
          
            sOut.println("AUTH LOGIN");
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("xxxxxxxxxxxxxxx=="); //username 
            sOut.println("xxxxxxxxxxxxxx="); //password 
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("MAIL FROM: <" + this.mailFrom + ">");
            this.nextSMTPCode(sIn, 250);
            sOut.println("RCPT TO: <" + destUser + ">");
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("DATA");
            this.nextSMTPCode(sIn, 250);
            sOut.println("From: " + this.mailFrom);
            sOut.println("To: " + destUser);
            sOut.println("Subject: " + subject);
            sOut.println(message);
            
            sOut.println(".");
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("QUIT");
            this.nextSMTPCode(sIn, 250);
            
            sIn.close();
            textMail = "From " + this.mailFrom + " To " + destUser + "\n" + "Subject: " + subject + "\nMessage: " + message;
            return true;
        } catch (IOException ex) {
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }

        return false;
    }

    public String getMail() {
        return textMail;
    }

    private void nextSMTPCode(BufferedReader sIn, int expectedCode) throws IOException {    
        int code;
        String str = sIn.readLine();
        System.out.println(str);
        String sCode = str.substring(0, 3);
        code = Integer.parseInt(sCode);

        if (code != expectedCode) {    
            sIn.close();
            System.out.println("Code: " + code + " expected code was: " + expectedCode);
            throw new IOException();
        }
    }
}

在与SMTP服务器（smtp2go）通信时遇到问题：即便在不该返回250码的场景下，也持续收到250响应码，最终导致邮件无法发送。

以下是一段通信示例：

220 mail.smtp2go.com ESMTP Exim 4.96-S2G Tue, 10 Jan 2023 14:42:02+0000
250-mail.smtp2go.com Hello  [xxxxx] 
250-SIZE 52428800 
250-8BITMIME 
250-DSN
250-PIPELINING 
250-PIPECONNECT 
250-AUTH CRAM-MD5 PLAIN LOGIN 
250-CHUNKING

之后发送的每一条命令，都会收到250-SIZE 52428800等带250码的响应。用telnet测试一切正常，问题显然出在代码中，但无法定位错误。

核心问题：未正确处理SMTP多线响应 #

SMTP服务器对EHLO等命令的响应是多线格式，每条行开头都是响应码（如250），但只有最后一行的响应码后不带连字符（-），表示响应结束。你的nextSMTPCode方法仅读取了第一行响应，剩余的多线内容会留在输入缓冲区中。后续发送命令后，程序读取的其实是之前EHLO响应的剩余行，而非新命令的真实响应，因此会持续收到250开头的内容。

修复步骤 #

1. 完善响应读取逻辑
   修改nextSMTPCode方法，循环读取所有响应行，直到找到不带连字符的目标响应码：

   private void nextSMTPCode(BufferedReader sIn, int expectedCode) throws IOException {    
       int code;
       String str;
       do {
           str = sIn.readLine();
           if (str == null) {
               throw new IOException("SMTP连接意外关闭");
           }
           System.out.println(str);
           String sCode = str.substring(0, 3);
           code = Integer.parseInt(sCode);
       } while (str.length() > 3 && str.charAt(3) == '-');
   
       if (code != expectedCode) {    
           sIn.close();
           System.out.println("收到响应码: " + code + " 预期响应码: " + expectedCode);
           throw new IOException();
       }
   }
   

2. 修正命令的预期响应码
   代码中多处预期响应码错误，这也是发送失败的关键：

   • AUTH LOGIN命令的正确响应码是334（提示输入用户名），发送用户名后再次收到334（提示输入密码），密码验证成功后返回235。
   • DATA命令的响应码是354（提示开始输入邮件内容），而非250。
   • EHLO命令需要携带本地主机名，不能只发送EHLO，应改为sOut.println("EHLO " + this.localMachine);。
   • QUIT命令的响应码是221，而非250。

   修正后的trySend方法相关片段：

   this.nextSMTPCode(sIn, 220);
   sOut.println("EHLO " + this.localMachine);
   this.nextSMTPCode(sIn, 250);
   
   sOut.println("AUTH LOGIN");
   this.nextSMTPCode(sIn, 334); // 等待用户名提示
   sOut.println("xxxxxxxxxxxxxxx=="); // 用户名Base64编码
   this.nextSMTPCode(sIn, 334); // 等待密码提示
   sOut.println("xxxxxxxxxxxxxx="); // 密码Base64编码
   this.nextSMTPCode(sIn, 235); // 认证成功
   
   sOut.println("MAIL FROM: <" + this.mailFrom + ">");
   this.nextSMTPCode(sIn, 250);
   sOut.println("RCPT TO: <" + destUser + ">");
   this.nextSMTPCode(sIn, 250);
   
   sOut.println("DATA");
   this.nextSMTPCode(sIn, 354); // 等待输入内容
   sOut.println("From: " + this.mailFrom);
   sOut.println("To: " + destUser);
   sOut.println("Subject: " + subject);
   sOut.println(); // 邮件头和正文之间需要空行
   sOut.println(message);
   
   sOut.println(".");
   this.nextSMTPCode(sIn, 250);
   
   sOut.println("QUIT");
   this.nextSMTPCode(sIn, 221);
   

3. 补充邮件格式细节
   邮件头和正文之间必须有一个空行，否则服务器可能无法正确解析邮件内容，这也是潜在的发送失败原因。

内容的提问来源于stack exchange，提问作者Conta