我的代码如下：
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();
}
}
编译运行正常，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意味着问题可能在服务器上，但是哪个服务器？
感谢任何帮助，谢谢
答案 0 :(得分：1)
错误由您正在与之通信的服务器mail.telkomsel.com发送。
为了防止垃圾邮件，大多数服务器都不接受发送带有域外邮件的电子邮件，而且没有正确的身份验证。错误代码421实际上意味着服务不可用。这是一种向垃圾邮件发送者说走开！的方式。