We aren't going to go into huge amounts of detail about running procmail, but just describe issues involving Qmail, SpamAssassin and TMDA.
By default, our Qmail installation delivers in Maildir format, to the directory
~/Maildir/ for each user. This is overridden on a user-by-user basis: Qmail will use a user's
~/.qmail file for delivery, if it exists. So for procmail, create
~/.qmail with the contents
|/usr/sbin/qmail-procmail
The construction of
qmail-procmail is described in the
procmail section of
Life with Qmail; its contents are described
here.
If the user's
.procmailrc contains something like
:0
/home/username/Maildir/
then procmail is basically operating as the identity file filter, delivering everything to the user's Maildir.