No messages in spam filter

[wellisch]

When I click on Spam filter training for any of my mail boxes Plesk reports that there are no messages in my mailbox. Yet there are hundreds of them in there.

InfoMessages learned: 0 as spam, 0 as non-spam, 0 total.
Messages in your mailboxNo message(s)

Can anyone tell me why this doesn't work or where I could look for further information?

Thank you!