[development] internal messaging among users for Drupal 6.x?

Hi folks! Not sure if this fits support or development...sorry for cross-posting. What is the logical solution to providing interface for internal messaging among users for Drupal 6.x? I need just basic inbox & outbox view, and compose, reply & forward forms. Is there a specific Module? something being ported from 5.x or made new for 6.x? Regards - Zeljko