[UA-EAI] Thoughts on Downgrading

[John Levine]

On Mon, 6 Nov 2017, Mark Svancarek wrote:

> Clarifying, since this question comes up from time to time:
>
> Also note that only the initial sender can do this.  If mail goes A->B->C, A can downgrade if the A->B hop can't do EAI, but B cannot if B->C can't
>
> Is this a MUST or a SHOULD (recognizing that RFC-style SHOULD is still very firm)?

MUST, absolutely, not negotiable.  B does not know what addresses A 
considers to be equivalent.

To clarify, we're talking about forwarders, not mailing lists.  Lists 
conceptually originate a new message (even if it looks a lot like the 
incoming message) and do all sorts of funky stuff to messages on the way 
through.

Regards,
John Levine, john.levine at standcore.com
Standcore LLC