Looks good Merk. Now the next step is to test spooling to a file, instead of maildir.
According to QMail docs, this is an easy step. It would be nice if you can test it, and let us know if it worked:
In the home directory of a test user account, create a file called .qmail the content of that file should be a single line indicating the spool file:
/var/spool/subscriber_notify
Or any path you choose. The file needs to exist, so we can first create by touch
touch /var/spool/subscriber_notify
Now send a test email to that account, and see if is directed into that file
I found the info here, from the Amail site:
http://www.qmail.org/man/man5/dot-qmail.html