by "identity", we refer to:
2821 HELO/EHLO domain
2821 MAIL FROM
2822 From:
2822 Sender:
There is a third way. The first way is for us to formulate some
prescriptive statement, endorsing "authentication" of one or more of these
"identities". The second way is a purely descriptive approach, describing
the outward MTA configuration for a domain (PHB, I think), and to be
agnostic on *use*.
A third way would allow a publisher to suggest (or even require) the
identities that the record should be applied to.