import java.net.*;
import java.io.*;
import java.util.*;
import javax.naming.*;
import javax.naming.Directory.*;

public class Smtp {

    public static void main(String[] args) throws Exception {
        // DNS服務器，看看本機的DNS配置
        String dns = "dns://192.168.1.1";
        // 郵箱后綴：
        String domain = "sohu.com";
        Hashtable env = new Hashtable();
        env.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
        env.put("java.naming.        DirContext ctx = new InitialDirContext(env);
        Attributes attr = ctx.getAttributes(domain, new String[]{"MX" });
        NamingEnumeration servers = attr.getAll();
        // 列出所有郵件服務器：
        while(servers.hasMore()) {
            System.out.println(servers.next());
        }
    }
}

第二步：直接連接郵件服務器的25端口，用SMTP協議發送郵件。
這里使用sohu信箱，郵件服務器為sohumx.sohu.com，收信人必須在此服務器上：

import java.net.*;
import java.io.*;
import java.util.*;
import javax.naming.*;
import javax.naming.directory.*;

public class Smtp {

    private static String END_FLAG = "/r/n";

    public static void main(String[] args) throws Exception {
        String mx = "sohumx.sohu.com";
        InetAddress addr = InetAddress.getByName(mx);
        Socket socket = new Socket(addr, 25);

        InputStream in = socket.getInputStream();
        OutputStream out = socket.getOutputStream();