I have spamsieve 3 installed i a computer with MacOS sonoma. I followed the instructions (junk mailbox is correctly assigned) but I keep on having several messages labelled [spam] in my inbox.
Am I missing something?
If you mean that the messages literally say [spam] in the subject, that means that a server junk filter classified the message as spam. SpamSieve may or may not agree, or it may not have looked at the message at all. You can tell by looking in the Log window for a Predicted log entry.
If SpamSieve thought it was spam but didn’t move it, there could be a problem with the Junk mailbox or Trash as configured in Mail, though it sounds like you already checked that.
If SpamSieve did not make a prediction for it, you may be running into a macOS bug, which you can work around by enabling Filter spam messages in other mailboxes and selecting your inboxes for filtering.