Barry Warsaw writes:
My own inclination is that most sites won't need this,
FWIW, I disagree. Much of the world is moving in the direction of personal IDs, perhaps backed up by an organization (eg, OpenID), rather than IDs tied to a host. Given the prevalence of systems "for the rest of us (who don't have a clue about PGP)", I doubt it will be a majority, but I bet you would get a lot of uptake if it were available.
so it should be available as a plugin,
Yes, it should be available as a Handler (or whatever the corresponding component of MM3 architecture is). It's not at all clear to me where in the stack of filters it belongs, and I can easily imagine people wanting to make it early or late. We won't know until we see the practice.
BTW, I hate that word, "plugin", which reminds of the hero of "Alice's Restaurant" getting "detected, inspected, injected, and neee-glected, with no part left untouched". Almost certainly you can be a lot more specific about *where* it fits into the architecture, such as "Handler".
As far as customizing the pipeline goes, it would be cool if there were a GUI which allowed moving/inserting/deleting Handlers (a la Emacs's "list" widget in Customize).