Exception being thrown when using the Commons-email API

I'm trying to use the Commons-Email API as an alternative to known JavaMail to send emails in Java but there is a problem. The following Exception is being released:

  

Exception in thread "main" java.lang.NoClassDefFoundError:   javax / mail / Message at Main.main (Main.java:6)

The problem occurs on line 6 when I try to instantiate an object of class SimpleEmail . Here is my code:

import org.apache.commons.mail.SimpleEmail;

public class Main {
    public static void main(String[] args) {
        try {
            SimpleEmail email = new SimpleEmail(); // ops?! Aqui é lançada a Exception
            email.setHostName("mail.foo.com");

            email.setFrom("[email protected]", "rnxn") // remetente
                 .addTo("[email protected]", "Sr. Foo") // destinatário
                 .setMsg("Exception sendo lançada ao utilizar a API Commons-email") // corpo do email
                 .setSubject("StackOverflow"); // título

            email.send(); // envio

        } catch(Exception e){
            System.out.println(e.getMessage());
        } 
    }
}

I've successfully imported all of the .jar files that came in the download. Any solutions to the treatment of this problem?