Right, and this produces one of the irrevocably broken downgrading scenarios:
Sender 1 has an EAI account ("U1") and wants to mail a message to both Receiver 2, who is EAI-enabled (account "U2") and also Receiver 3, who is not (account "A3"). Sender has a downgrading email provider who has also assigned alias
A1 to the same mailbox as U1.
Receiver 2 sees:
From:U1 Cc:A3
Receiver 3 sees:
From:A1 Cc: <implementation-specific*>
If Receiver 2 ReplyAll, Sender 1 sees:
From:U2 Cc:A3
Receiver 3 doesn't see anything and Receiver 2 gets a nondelivery notice.
Even if Receiver 2 also has a downgrading email service, it's not allowed for the "U1" address to be converted to something that Receiver 3 can accept!
* NOTE: Depending on the implementation-specific details (did Sender 1 convert "U2" into ACE? And if so, what did Receiver 3 do with it?), there may be additional breaks. Just keep remembering that downgrading is always imperfect and
that the primary scenarios are people emailing to others who use their same script, and it's not so bad.
Ajay, do you want to weigh in on this?
/marksv
-----Original Message-----
From: John Levine [mailto:john.levine@standcore.com]
Sent: Monday, November 6, 2017 4:54 PM
To: Mark Svancarek <marksv@microsoft.com>
Cc: ua-eai@icann.org
Subject: RE: [UA-EAI] Thoughts on Downgrading
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@standcore.com
Standcore LLC