Hi,
You may wish to have a look in a page I've put here:
FastMail Settings
It details instructions with screenshots for accessing FastMail with a Psion 5mx, using different mail clients and web browsers.
Please excuse the adverts that lycos/tripod display on the page - it's free so I have had to put up with it
I noticed that in order for the subfolders to be accessible, I have to leave the Root folder/path blank. Otherwise I can only see the Inbox.
Let me know if you need additional information.
Kind regards,
Mike