John Levine wrote > Again, I don't see how that contstrains the address beyond > requiring that it accept bounces.
Standard quote: "the originator as indicated in the reverse path". It *identifies* the originator, it's not designed to be something like a Message-ID. As Ned said, this is a major deviation from what an envelope sender address used to be for some decades, and it's IMO not *obvious* how it works in conjunction with RFC 3834 + SIEVE. Likely it does, otherwise you'd know these problems already, but you could have missed obscure issues with the completely new semantics. Frank
