Check your junk filter settings. https://support.mozilla.org/kb/thunderbird-and-junk-spam-messages

Hello christ1, thanks for your reply. But I had already read that before I asked you.

My problem is that I have two folders (Junk and Suspected Spam) and don't know how they work together. This interaction is not described in the instructions (your link) either.

The junk filter will probably load into the Suspected Spam folder. After working through the emails there, there are now emails with and without junk flags. But what happens then?

With best regards

What is your account type - POP or IMAP?

My account type is IMAP.

My problem is that I have two folders (Junk and Suspected Spam) and don't know how they work together.

That's one too many. You'll have to find out to which folder messages classified as 'Junk' will be sent to. A pragmatic way to test this is to mark a message as 'Junk', and check where it goes to. You may also check your account via webmail to determine which folder is the one used by your email provider's server. What makes things more difficult is a bug introduced recently, where Thunderbird shows the spam folder used by your email provider's server, e.g. 'Suspected Spam', just as 'Junk'. That bug will be fixed in TB 144 to be released in about two weeks time. So if you're unsure about the correct folder you may just wait until then.