Invoke script on existing inbox messages?

        Hi,

        I've recently started teaching myself procmail -- been getting
        this list for about a week.  Now I'd like to be able to 
        test some scripts (recipes) on my existing inbox (force procmail
        to sort my existing messages according to a particular rc file).

        I saw a post that seemed to implay  that this is possible --
        forays through the man pages haven't helped clear it up for
        me.

        So, how does one do this?