This is the initial release of the Drupal 6.x compatibility port.
You may consider this a release candidate. Depending on the feedback I get, I will tag it 1.0 in a week or two (it was released July 6, 2008).
There have been many changes to the code, not just including the porting. I removed the included phpmailer and smtp classes. You now have to download the source (information is included in README.txt and INSTALL.txt). I've also cleaned up the code to be in line with Drupal Coding Standards.
Please try it out and leave feedback in the issue queue: http://drupal.org/project/issues/smtp.
I wish to give thanks to all the people who kept this module alive in Drupal 6 while I was creating this. The issue is http://drupal.org/node/214975.