Hi The following code causes an error. Please help me understand what's wrong.

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
public class SendMail
{
  public static void main(String [] args)throws MessagingException
  {
    SendMail sm=new SendMail();
     sm.postMail(new String[]{"[email protected]"},"hi","hello","[email protected]");
   }

public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
{
    boolean debug = false;

     //Set the host smtp address
     Properties props = new Properties();
     props.put("mail.smtp.host", "webmail.emailmyname.com");

    // create some properties and get the default Session
    Session session = Session.getDefaultInstance(props, null);
    session.setDebug(debug);

    // create a message
    Message msg = new MimeMessage(session);

    // set the from and to address
    InternetAddress addressFrom = new InternetAddress(from);
    msg.setFrom(addressFrom);

    InternetAddress[] addressTo = new InternetAddress[recipients.length]; 
    for (int i = 0; i < recipients.length; i++)
    {
        addressTo[i] = new InternetAddress(recipients[i]);
    }
    msg.setRecipients(Message.RecipientType.TO, addressTo);


    // Optional : You can also set your custom headers in the Email if you Want
    msg.addHeader("MyHeaderName", "myHeaderValue");

    // Setting the Subject and Content Type
    msg.setSubject(subject);
    msg.setContent(message, "text/plain");
    Transport.send(msg);
}
}

Exception:

Exception in thread "main" javax.mail.MessagingException: Could not connect to SMTP host: webmail.emailmyname.com, port: 25, response: 421
    at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1694)
    at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:525)
    at javax.mail.Service.connect(Service.java:291)
    at javax.mail.Service.connect(Service.java:172)
    at javax.mail.Service.connect(Service.java:121)
    at javax.mail.Transport.send0(Transport.java:190)
    at javax.mail.Transport.send(Transport.java:120)
    at SendMail.postMail(SendMail.java:45)
    at SendMail.main(SendMail.java:9)