1. it's a non-starter to force senders to submit to/relay through any
particular host or hosts. people can and do send mail from
anywhere, and this is essential functionality.
Providing a list of authorized senders is a sender-side choice. As such,
this is an elective system and has to be recognized as one.
right. but it's an elective system that has significant barriers to
actually being used.
BUT, providing this kind of information is as beneficial to the sender as
it is to the recipient.
right, but it only works for senders who always use the same relays.
So rather than looking at this in terms of ~"can I reject mail from
senders without MS RRs?" (the answer to that is "no"), the appropriate way
to look at this is ~"can the owner of a domain use this to prevent
forgeries from their domain" and "can the recipients use this as a
supplemental method for killing spam" (the answer to both of those
questions is "yes").
the answer to the first of the questions is "no, not in general".
the answer to the second question is "yes, but it won't make much
of a difference unless most of the sources of spam use it"
2. my guess is that IESG isn't likely to approve a new use of, or
extensions to, the MS record.
Maybe you could be bothered to provide some kind of support for this.
it's just my guess from having been on IESG.
Keith