Sending e-mail: EventDispatchThread.pumpOneEventForHierarchy

Can you paste the entire exception stack trace?

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: javax/activation/DataSource
      at mail.MailSender.<init>(MailSender.java:26)
      at mail.MainFrame.actionPerformed(MainFrame.java:71)
      at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
      at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
      at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
      at java.awt.Component.processMouseEvent(Unknown Source)
      at javax.swing.JComponent.processMouseEvent(Unknown Source)
      at java.awt.Component.processEvent(Unknown Source)
      at java.awt.Container.processEvent(Unknown Source)
      at java.awt.Component.dispatchEventImpl(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Window.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.EventQueue.dispatchEvent(Unknown Source)
      at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.run(Unknown Source)

In case of Eclipse, to the build-path of the project you are running by Properties -> Java Build Path -> Add External JARs

Thanks! I still have another question, maybe you can help me:

I tried to create and send my email with the following code, but get always the message:
Fehler in Methode sendMessage(): 550 5.7.0 Need to authenticate via SMTP-AUTH-Login {mp038}

com.sun.mail.smtp.SMTPSendFailedException: 550 5.7.0 Need to authenticate via SMTP-AUTH-Login {mp038}

      at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:1388)
      at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTransport.java:959)
      at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:583)
      at javax.mail.Transport.send0(Transport.java:169)
      at javax.mail.Transport.send(Transport.java:98)
        .....


Properties props = System.getProperties();
                    MyAuthenticator auth = new MyAuthenticator();
                    props.put("mail.smtp.host", "mail.gmx.net");
                    // holt Referenz auf ein Session-Objekt
                       Session mailSession = Session.getDefaultInstance(props, auth);
                      
                      msg = new MimeMessage(mailSession);
                       // erzeugt ein MimeMessage-Objekt
                       
                  // Im folgenden werden die Absenderadresse, der direkte
                  // Empfänger, das Absendedatum, der Betreff kodiert in
                  // US-ASCII Zeichen und der Headereintrag "X-Mailer" gesetzt
                  msg.setFrom(new InternetAddress(from));
                  msg.setRecipients(Message.RecipientType.TO,
                        InternetAddress.parse(to, false));
                  msg.setSentDate(new Date());
                  msg.setSubject(
                MimeUtility.encodeText(subject,"iso-8859-1","Q"));
                  msg.setHeader("X-Mailer", "JavaMail");

                  // kein Anhang, Mailtext wird direkt der Mail hinzugefügt.
                  msg.setText(messageText, "iso-8859-1");
                  
                  Transport.send(msg);

class MyAuthenticator extends Authenticator {  
            protected PasswordAuthentication getPasswordAuthentication() {
                  
                  return new PasswordAuthentication ("MyAdr@gmx.de", "myGmxPwd");
            }
      }