Smart Mailbox filter "Message is not in mailbox" for iCloud mailboxes

Solution 1:

I had the same problem and I found a solution here luckily after searching like crazy. The problem was, that I used my Gmail account as my Apple ID, so mail got confused. After I deactivated mail in my iCloud account settings, it was working fine. This is obviously only a solution if you don't use both accounts. Anyway, maybe it helps someone:

And here's the solution, or rather the explanation for the problem: If you're using your gmail Adress as your apple-id (and therefore as your login credentials for your iCloud account) Apple Mail seems to get confused. Simply change your iCloud Username to something else and your good to go.

Solution 2:

I don't know how to fix it, but I have a workaround. Create a smart mailbox (we'll call it Workaround), set it to "message is in mailbox" and then select the iCloud mailbox you want. Then create a second smart mailbox and set it to "message is not in mailbox" - "Workaround". This seems to work for me on High Sierra. Now you have an extra smart mailbox, but it gets the job done.