用javamail发邮件，邮件服务器名称怎么设置？

//  java  mail  package  http://java.sun.com/products/javamail/index.html
//java  mail  depends  on  http://java.sun.com/products/javabeans/glasgow/jaf.html
//Set  your  CLASSPATH  to  include  the  "mail.jar"  file  obtained  from
//          the  download,  as  well  as  the  current  directory.
//          Assuming  you  unzipped  javamail-1_2.zip  in  c:\download  the
//          following  would  work:
//            set  CLASSPATH=%CLASSPATH%;c:\download\javamail-1.2\mail.jar;.
//        Also  include  the  "activation.jar"  file  that  you  obtained  from
//        downloading  the  JavaBeans  Activation  Framework,  in  your  CLASSPATH.
//            set  CLASSPATH=%CLASSPATH%;c:\download\activation\activation.jar
import  javax.mail.*;
import  javax.mail.internet.*;
import  java.util.*;
import  java.io.UnsupportedEncodingException;
import  javax.swing.*;
import  java.awt.*;
import  java.awt.event.*;
public  class  Mail  extends  JFrame
{
    JLabel  jlSendMail  =  new  JLabel("Send  Mail");
    public  Mail()
    {
        this.getContentPane().setLayout(new  FlowLayout());
        this.getContentPane().add(jlSendMail);
        jlSendMail.addMouseListener(new  MouseAdapter()
        {
            public  void  mouseClicked(MouseEvent  me)
            {
                try{
                    String  host  =  "smtp.21cn.com";//"Mail  Server  in  your  Company";
                    String  from  =  "[email protected]";//"From  Email";
                    String  to  =  "[email protected]";//"To  Email";
                    Properties  props  =  new  Properties();
                    Session  session;
                    //Store  store;
                    Transport  transport;
                    props.put("mail.smtp.host",  host);
                    session  =  Session.getInstance(props,  null);
                    //session.setDebug(true);
                    Message  message  =  new  MimeMessage(session);
                    message.setFrom(new  InternetAddress(from,"Ahmad"));
                    message.setRecipient(Message.RecipientType.TO,  new  InternetAddress(to,"Ahmadwa"));
                    message.setSubject("First");
                    //message.setSentDate(new  Date());
                    message.setText("Hi  Emad"  +  (char)(10)  +  "How  are  you?  This  is  a  test  mail  send  by  java  program"+new  Date());
                    transport  =  session.getTransport("smtp");
                    transport.send(message);
                    System.out.println("Your  Mail  Send  Successfully");
                }
                catch  (MessagingException  e)  {System.out.println("1)"  +  e.toString());}
                catch  (UnsupportedEncodingException  e)  {System.out.println("2)"  +  e.toString());}
            }
        });
        this.setSize(600,600);
        this.setVisible(true);
    }
    public  static  void  main(String  args[])
    {
        new  Mail();
    }
}