Drupal 5 Backport

Unfortunately no. Backporting Webmail for 6.x would require a tremendous effort since it relies on a number of components that aren't available for 5.x.

There is the Basic webmail module that does work for Drupal 5.x. Because the Webmail Plus module is so much more advanced already, and does basically the same thing, there are no plans for upgrading the Basic webmail module to Drupal 6.x.

Closing an old issue.