autoresponding to Yahoo DMARC breakage

[morrowc.lists at gmail.com (Christopher Morrow)]

On Wed, Apr 9, 2014 at 6:27 PM, John R. Levine <johnl at iecc.com> wrote:
>>> The most "sane" out-of-mind response should only be sent *if* the
>>> out-of-mind person is named explicitly as a recipient in the RFC822
>>> To: header.  Anything To: somelist at somehost does not qualify :)
>
>
> This highly effective trick was in the procmail example vacation script in
> 1991, and doubtless goes back much farther than that.  It's a little
> dismaying to hear that there are still people writing autoresponders who
> don't know about it.

what is procmail?