Thank you for replying! Relaying does seem like a fatal flaw. DNS seemed like a bad match for per-user information (rather than per-domain), but indeed that HOW

Thank you for replying! Relaying does seem like a fatal flaw. DNS seemed like a bad match for per-user information (rather than per-domain), but indeed that HOWTO gives a nice way of working around that. So it's just a question of MUA PGP modules trying that mechanism.