java - 我的 SMTP 发件人抛出错误 421

Question

我的代码工作原理如下:

import java.io.*;
import java.net.*;
import java.util.*;

public class SMTPClientDemo {
  protected int port = 25;
  protected String hostname = "localhost";
  protected String from = "";
  protected String to = "";
  protected String subject = "";
  protected String body = "";
  protected Socket socket;
  protected BufferedReader br;
  protected PrintWriter pw;

  public SMTPClientDemo() throws Exception
  {
    // TODO code application logic here
    try
    {
        getInput();
        sendEmail();
    }
    catch (Exception e)
    {
        System.out.println ("Error sending message - " + e);
    }
 }
   public static void main(String[] args) throws Exception {
    // TODO code application logic here
    // Start the SMTP client, so it can send messages
    SMTPClientDemo client = new SMTPClientDemo();
 }
// Check the SMTP response code for an error message
protected int readResponseCode() throws Exception
{
    String line = br.readLine();
    System.out.println("< "+line);
    line = line.substring(0,line.indexOf(" "));
    return Integer.parseInt(line);
}

// Write a protocol message both to the network socket and to the screen
protected void writeMsg(String msg) throws Exception
{
    pw.println(msg);
    pw.flush();
    System.out.println("> "+msg);
}

// Close all readers, streams and sockets
protected void closeConnection() throws Exception
{
    pw.flush();
    pw.close();
    br.close();
    socket.close();
}

// Send the QUIT protocol message, and terminate connection
protected void sendQuit() throws Exception
{
    System.out.println("Sending QUIT");
    writeMsg("QUIT");
    readResponseCode();
    System.out.println("Closing Connection");
    closeConnection();
}

// Send an email message via SMTP, adhering to the protocol known as RFC 2821
protected void sendEmail() throws Exception
{
    System.out.println("Sending message now: Debug below");
    System.out.println("-------------------------------------");
    System.out.println("Opening Socket");
    socket = new Socket(this.hostname,this.port);
    System.out.println("Creating Reader & Writer");
    br = new BufferedReader(new
    InputStreamReader(socket.getInputStream()));
    pw = new PrintWriter(new
    OutputStreamWriter(socket.getOutputStream()));
    System.out.println("Reading first line");
    int code = readResponseCode();
    if(code != 220) {
        socket.close();
        throw new Exception("Invalid SMTP Server");
    }  
    System.out.println("Sending helo command");
    writeMsg("HELO "+InetAddress.getLocalHost().getHostName());
    code = readResponseCode();
    if(code != 250)
    {
        sendQuit();
        throw new Exception("Invalid SMTP Server");
    }
    System.out.println("Sending mail from command");
    writeMsg("MAIL FROM:<"+this.from+">");
    code = readResponseCode();
    if(code != 250)
    {
        sendQuit();
        throw new Exception("Invalid from address");
    }
    System.out.println("Sending rcpt to command");
    writeMsg("RCPT TO:<"+this.to+">");
    code = readResponseCode();
    if(code != 250)
    {
        sendQuit();
        throw new Exception("Invalid to address");
    }
    System.out.println("Sending data command");
    writeMsg("DATA");   
    code = readResponseCode();
    if(code != 354)
    {
        sendQuit();
        throw new Exception("Data entry not accepted");
    }
    System.out.println("Sending message");
    writeMsg("Subject: "+this.subject);
    writeMsg("To: "+this.to);
    writeMsg("From: "+this.from);
    writeMsg("");
    writeMsg(body);
    code = readResponseCode();
    sendQuit();
    if(code != 250)
    throw new Exception("Message may not have been sent correctly");
    else
    System.out.println("Message sent");
}

// Obtain input from the user
protected void getInput() throws Exception
{
    // Read input from user console
    String data=null;
    BufferedReader br = new BufferedReader (new InputStreamReader(System.in));

    // Request hostname for SMTP server
    System.out.print("Please enter SMTP server hostname: ");
    data = br.readLine();
    if (data == null || data.equals("")) hostname="localhost";
    else
    hostname=data;
    // Request the sender's email address
    System.out.print("Please enter FROM email address: ");
    data = br.readLine();
    from = data;
    // Request the recipient's email address
    System.out.print("Please enter TO email address :");
    data = br.readLine();
    if(!(data == null || data.equals("")))
    to=data;
    System.out.print("Please enter subject: ");
    data = br.readLine();
    subject=data;
        System.out.println("Please enter plain-text message ('.' character on a blank line signals end of message):");
        StringBuilder buffer = new StringBuilder();
    // Read until user enters a . on a blank line
        String line = br.readLine();
        while(line != null)
        {
            // Check for a '.', and only a '.', on a line
            if(line.equalsIgnoreCase("."))
            {
                break;
            }
            buffer.append(line);
            buffer.append("\n");
            line = br.readLine();
        }
        buffer.append(".\n");
        body = buffer.toString();
    }

}

编译顺利，当我尝试运行它时，结果是:

 Please enter SMTP server hostname: mail.telkomsel.com
 Please enter FROM email address: andikacj@aim.com
 Please enter TO email address :kamikaze273@gmail.com
 Please enter subject: coba
 Please enter plain-text message ('.' character on a blank line signals end of message):
 coba14
 .
 Sending message now: Debug below
 -------------------------------------
 Opening Socket
 Creating Reader & Writer
 Reading first line
 < 220 ESMTP mail.telkomsel.com
 Sending helo command
 > HELO Cruzer-PC
 < 250 mailpapp1.telkomsel.co.id
 Sending mail from command
 > MAIL FROM:<andikacj@aim.com>
 < 250 2.1.0 Ok
 Sending rcpt to command
 > RCPT TO:<kamikaze273@gmail.com>
 < 250 2.1.5 Ok
 Sending data command
 > DATA
 < 354 end data with <CR><LF>.<CR><LF>
 Sending message
 > Subject: coba
 > To: kamikaze273@gmail.com
 > From: andikacj@aim.com
 > 
 > coba14
 .

 < 421 syntax error (wait for server response)
 Sending QUIT
 > QUIT
 < null
 Error sending message - java.lang.NullPointerException

我可以保证的是，421 意味着问题可能出在服务器上，但是是哪台服务器？

任何帮助将不胜感激，谢谢

Answer 1

该错误是由您正在通信的服务器 mail.telkomsel.com 发送的。

为了防止垃圾邮件，大多数服务器不会接受发件人地址位于其域之外并且未经适当身份验证的电子邮件。错误代码 421 实际上意味着服务不可用。这是对垃圾邮件发送者说走开!的一种方式。