What would be more/also interesting would be a way of expressing this intent by a sender. Seems like (with absolutely no thought here) a new header could me mi

What would be more/also interesting would be a way of expressing this intent by a sender. Seems like (with absolutely no thought here) a new header could me minted for this - eg. "Transport-Security: Require". Tough part as usual would be getting MTA and client support, but switching to a per message approach would at least allow incremental rollout - and critically I think - allow introspection by following bounces.