It's possible to subscribe/unsubscribe to folders in your webmail. If you for example receive a mail about your quota has been used and you've deleted mails, but there's still not space. Then it can sometimes be because there are mails in "hidden" folders. To find these folders, you have to do the following:
1. Open your webmail
2. Login with email and passphrase
3. Click the cogwheel to access your settings:
4. Click on the "eye" next to the folders you want to see:
Dark grey - Means that the folder is shown
Light grey - Means that the folder is not shown
5. When you're done, you just press the "Back" button.