I’ve changed the system procmail rules as follows:

DROPPRIVS=yes
MAILDIR=$HOME/mail
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc
:0:
* ? test ! -f $HOME/.procmailrc
* ^X-Spam-Status: Yes
spam

The reason for this change is so the system procmail rules won’t put mail scored as spam in a spam folder with giving user procmail rules a chance to override.

If you have a .procmailrc file and want mail scored as spam to go into a spam folder, then you need a rule like this:

:0:
* ^X-Spam-Status: Yes
spam

If you’d rather have mail that scores as spam discarded, then you need a rule like this:

:0:
* ^X-Spam-Status: Yes
/dev/null

If you have no .profmailrc file, then the default behavior is still to put mail marked as spam in your spam folder.