I use it like this:
/extern.php?action=feed&type=rss
You can see the problem If you look at the source code of the generated feed 
1 2009-06-17 13:43
Re: RSS Feed (extern.php) - partially wrong content (4 replies, posted in PunBB 1.3 bug reports)
2 2009-06-16 10:34
Topic: RSS Feed (extern.php) - partially wrong content (4 replies, posted in PunBB 1.3 bug reports)
- The <author> field contains "dummy@example.com" as mail address..
- The author name is somethimes completely wrong (seen with the "main" freed of all forums/threads).