All JFrame freeze while do JavaMail

When you do heavy task you should run them in another threads rather in the same as gui. If you run in Event Dispatch Thread then the gui is gonna freeze until finish.

You can use SwingWorker here is an example i really like Swing Worker Example

Example:

class Worker extends SwingWorker<String, Object> {

    @Override
    protected String doInBackground() throws Exception {
       //here you send the mail
       return "DONE";
    }

    @Override
    protected void done() {
        super.done();
        //this is executed in the EDT
        JOptionPane.showMessageDialog(null, "Message sent", "Success", JOptionPane.INFORMATION_MESSAGE);
    }
}