The content of the main qmail file ~/.qmail-default is a mailaddress

id:qmail-default-content-mailaddr

The file ~/.qmail-default is the only qmail file neccessary for the standard mail setup on U7.

When the file is setup with a mailaddress, this will forward all incoming mails to this address and bypasses the standard mail system without filtering spam. You can use instead a real catchall with a mailbox.

First you need to remove the current file and restore the standard mail setup:

[isabell@stardust ~]$ rm ~/.qmail-default
[isabell@stardust ~]$ uberspace mail spamfolder enable
The spam folder is now enabled.

Then lookup the manual how to create a catchall.

Error

Error: You will need to fix the mailsetup before you can fix any of your individual mailaddress configurations.