From the qmail FAQ (4.4 How do I use procmail with qmail?)
Put
| preline procmail
into ~/.qmail. You'll have to use a full path for procmail
unless procmail in in the system's startup PATH. Note that
procmail will try to deliver to /var/spool/mail/$USER by
default; to change this, see INSTALL.mbox.
I've done a little searching and haven't found it yet. I
was wondering if
there was someone on the list who can point me in the
direction of a FAQ
which details installation of procmail with qmail. I can't
seem to tweak
procmail enough to get it to work with qmail.