Subject: Re: [Emailcore] Delivered-To header

From: Hector Santos <[email-masked]>
Date: Sat, 02 Jan 2021 11:24:47 -0500

On 12/31/2020 10:55 AM, John R Levine wrote:
> RFC5321 says that when a message is delivered the envelope MAIL FROM
> address goes into a Return-Path header, but says nothing about the
> RCPT TO address.

For Return-Path, only for a relay/route, not for the MDA.  In our 
system, the MDA/SMTP Gateway has no need for the Return-Path and thus, 
it is removed which is carry on code from the UUCP Gateway days of 
removing the "From " field (note no colon) once final destination is 
established.

> Many MTAs put that in a Delivered-To header and have
> done so for a long time.  I know that Postfix and qmail do.

Yet wcSMTP and others do not. Not a justification. Tracing the target 
address can be and is done with the initial SMTP 5322.Received trace 
line.

> I think this is sufficiently established

"Sufficiently established?" Without getting input from all other SMTP 
developers?

> ... that we can add it to
> 5321bis. We could say that the delivery SMTP server SHOULD insert a
> Delivered-To line when it inserts a Return-Path line, and SHOULD NOT
> remove existing Delivered-To lines.

-1 for a SHOULD. Maybe +1 with a small "may" or "could" but lets keep 
in mind we already have a 5322.Received header that resolves this 
tracer bit, for example for the OP message:

Received: by mail.winserver.com (Wildcat! SMTP Router v8.0.454.10)
           for [email-masked]; Thu, 31 Dec 2020 10:55:39 -0500

A SHOULD brings back a reinforcement consideration to a "To:" field 
that was relaxed when we went from RFC822 to RFC2822.  Also consider 
some SMTP software MAY add a 5222.To if missing.  If the 5322.To field 
is not required, than neither should 5322.Delivered-To.

-- 
Hector Santos,
https://secure.santronics.com
https://twitter.com/hectorsantos