[Apologies if I'm reinterating anything that's already been discussed;
I've skimmed the archives but may have missed something...]
Since I haven't seen this mentioned except in passing, I'd like to
suggest that per-user policies are important, and explain why I
believe this to be the case.
SPF appears to be the only existing proposal that offers this; I'm
well aware that this adds complexity but I think the functionality is
important to the adoption of any MARID scheme that designatates valid
originating IP addresses.
The problem is that such schemes require changes in user behaviour.
They mean that users can no longer send mail with their 821 identity
through an SMTP relay of their choice. There are considerations when
setting up forwarding to their addresses. Therefore it requires a
policy change in any organisation that chooses to publish MARID
records.
Many large organizations have a flat e-mail address space of the form
user(_at_)organization(_dot_)tld, although the organization itself will of
course
not be administratively flat. In such an organization, without
per-user policy, adopting MARID and the necessary changes in user
behaviour will be an all-or-nothing decision, and I fear
organizational politics will get in the way.
In such an organization, publishing MARID records will require a
change in organizational policy, rather than just changes in
departmental policies. It just requires one department head to argue
that his users don't have time to deal with this to block adoption
across the entire organization. Per-user policy allows piecemeal
adoption on a per-department basis, and is therefore more likely to
happen.
What a per-user policy mechanism should look like, I don't know. But
my initial feeling is that you need something similar to the SPF macro
mechanism in order to be sufficiently expressive to cope with the
majority of cases. (Mainly because of the fact that localparts are
nominally opaque, and hence MARID can't make too many assumptions
about how they might be structured.)
My gut feeling is that the benefits are worth the price, because it
will aid rapid adoption in large organizations.
-roy