Jar file could not find main class.. I cant see why?

Question

I have read loads of tutorials on this and it should be very straight forward but i STILL cant make a working jar file of my program.. please see if you can see anything wrong with what im doing..
my program is one .java file that compiles perfectly, and is runnable both through eclipse and by using the command prompt.. i made a jar file using both eclipse and the command prompt, and neither worked.. every time i run it by double clicking it i get the error "could not find main class, program will exit". here is the contents of my jar:
CorePackage\IPEmailer.class
CorePackage\IPEmailer$1.class
CorePackage\IPEmailer$SMTPAuthenticator.class
META-INF\MANIFEST.MF
the MANIFEST.MF file contains:
Manifest-Version: 1.0
Class-path: .
Created-By: 1.6.0_02 (Sun Microsystems Inc.)
Main-Class: CorePackage.IPEmailer
(yes there is a blank line at the end)
the class mentioned in 'Main-Class' has a main method that looks like this:
public static void main(String[] args) {
    // TODO Auto-generated method stub
    new IPEmailer();
}the program uses 2 external jar files:
activation.jar
mail.jar
and reads a file called:
ipemailer.ini
and writes a file called:
ip.txt
the program has no GUI and is intended to run 'silently' in the background
please help me understand what is wrong with this

Answer 1

package CorePackage;
import javax.mail.*;
import javax.mail.Authenticator;
import javax.mail.PasswordAuthentication;
import javax.mail.internet.*;
import java.util.*;
import java.io.*;
import java.net.*;
public class IPEmailer implements Runnable{
      * @param args
     String recipientsinput[]={""};
     String subject;
     String body;
     String server;
     String user;
     String pass;
     String from;
     String urlString = "http://www.whatismyip.org";
     String currentIP;
     String ipFromFile;
     boolean webFailed;
     long interval = 900000; //15 mins
     long oldInterval;
     Thread t = new Thread(this);
     public static void main(String[] args) {
          // TODO Auto-generated method stub
          new IPEmailer();
     public IPEmailer(){
                     ..........etccould the problem be that the class is a thread? i cant see why it would be, but ive never made a jar where the main class also 'implements runnable'