From a technical standpoint something like that could work to a limited extent (enough to be helpful in some situations, when both parties are using modern, up

From a technical standpoint something like that could work to a limited extent (enough to be helpful in some situations, when both parties are using modern, up to date MTAs / mail clients). It still doesn't fix the error message from a bounce, it's more like a hint to the email author to say "this email might not be accepted for the following reasons: x, y and z". Ultimately though, the inertia to implement such a thing on existing MTAs is somewhere close to the difficulty of implementing IPv6. Except the need for IPv6 is verging on dire, and not many people seem to be rushing to fix that one. How will we convince people to act with any expediency over something that is, at most, an inconvenience?