Actually, searching for MXPS (thank-you) in the archives, I found:
http://www.ornl.gov/its/archives/mailing-lists/qmail/1999/01/msg00791.html
... by DJB (in January, 1999):
-X-
I'm going to use a special MX host name format instead of special MX
preferences. The basic options will be
_magic.s.* I can receive mail by SMTP
_magic.q.* I can receive mail by QMTP
_magic.qs.* I can receive mail by QMTP or SMTP
with the possibility of future extensions such as
_magic.abcdqrsz.*
-X-
James Raftery wrote:
> On Mon, Jul 24, 2000 at 05:32:17PM -0400, Michael T. Babcock wrote:
> > DJB mentions on his 'future of qmail' page that a way to encode that a
> > host supports QMTP into its MX data is in the works. What method for
> > doing so is proposed?
>
> http://cr.yp.to/proto/mxps.txt, I imagine.
