Re: [ietf-smtp] homework, not an experiment, draft-crocker-email-deliver

On Wed, Aug 04, 2021 at 07:39:13AM -0700, Dave Crocker wrote:
> On 8/4/2021 7:11 AM, Viktor Dukhovni wrote:
> > The main thing that's implicit is that MUAs can't generally expect
> > to find a usable recipient address in the recorded "Delivered-To"
> > mailbox.  This may be true for some operators for some period of
> > time, but is not a requirement of the field semantics.
>
> I believe that concern is outside the scope of this draft.  And I don't 
> think there is any text in the draft that is relevant to it; nor does it 
> seem necessary to add any.
>
> No?
The abstract seems to me to suggest semantics other than an MTA-private
loop sentinel:

   The address to which email is delivered might be different than any
   of the addresses shown in any of the content header fields that were
   created by the email author.  The address used by the email transport
   service is provided separately, through an envelope SMTP "RCPT TO"
   command.  Before final delivery, handling can entail a sequence of
   submission/delivery events, using different destination addresses,
   that lead to the recipient.  It can be helpful for a message to have
   a common way to record each delivery in such a sequence, and to
                   -----------------------------------------------
   include each address used for that recipient.  This document defines
   ---------------------------------------------
   a header field for this information.

The sentinels used by MTAs in "Delivered-To" are used for loop
detection.  Use of the actual delivery address is merely the
simplest way to encode the requisite data.

It therefore seems that the goal of the draft is to *require* the field
to carry the actual delivery address, so that tools such as fetchmail
could then rely on its syntax and semantics.  If so, an MTA that wants
to preserve delivery address privacy, and still do loop detection, might
then need to switch to a suitable alternative header.

I might note that the delivery address for a recipient is an internal
matter between the MTA/LDA, that might change over time even for the
same underlying recipient.  So MUAs should not expect long-term
stability for such addresses.

Finally, the "Experimental" status is rather odd, given that the header
has been in use for over two decades...

-- 
    Viktor.

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