Unfortunately notifications has zero documentation 😟
The best you can do is to look into the code at notifications/tcl/reply-sweep-procs.tcl to get an idea how it works. What I can remember is that you need to activate the feature by setting a parameter of the notifications package (in the site-map), and dedicate a Maildir directory (it must be in Maildir format - mbox etc. won't work).
Make sure the aolserver process has read and write access to the Maildir directory and to new incoming messages as well (qmail insists to create them with -rw------- permission, so you should propably tell it to deliver them as the aolserver user).
After activating it a sweeper proc will run periodically to look for new incoming mails in the dedicated maildir. tail -f the error log to see what happens when it doesn't work as expected.
If you don't want to wait for the sweeper proc to be activated automatically while debugging it is advisable to create a script with which you can start it manually, e.g. directly under serverroot/www/test.tcl, that simply contains the line notification::reply::sweep
