I am getting run time error

 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);
    }
    }


Error
Exception in thread "main" java.lang.NoClassDefFoundError: SendMail
Caused by: java.lang.ClassNotFoundException: SendMail
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: SendMail.  Program will exit.