Re: Mailing list usage questions

[David Wright <deblis@lionunicorn.co.uk>]

On Wed 12 Apr 2023 at 14:02:39 (-0400), Jeffrey Walton wrote:
> On Wed, Apr 12, 2023 at 1:52 PM <tomas@tuxteam.de> wrote:
> > On Wed, Apr 12, 2023 at 12:38:37PM -0400, Jeffrey Walton wrote:
> > > On Tue, Apr 11, 2023 at 10:12 PM The Wanderer <wanderer@fastmail.fm> wrote:
> > > > ...
> > > > Some mail services apparently treat this "discard incoming messages that
> > > > look like duplicates of ones you already have a copy of" behavior as a
> > > > feature; Gmail is the best-known example. That has problems when (as
> > > > with this mailing list) the incoming copy is not identical to the one
> > > > that was sent, even though it has the same Message-ID, but AFAIK they
> > > > don't seem to care.
> > >
> > > I believe Message-Id's are supposed to be unique across space and
> > > time. It sounds like discarding the duplicate is expected behavior (to
> > > me).
> >
> > Still it doesn't make much sense discarding incoming messages which
> > match the IDs of outgoing ones.
> 
> Most (nearly all?) mail agents follow the RFCs. The RFCs say a
> Message-Id is unique across space and time. A duplicate Message-Id
> means a duplicate message.
> 
> If you wish to wander from the convention, then don't be surprised
> when unexpected things happen.

  Note: There are many instances when messages are "changed", but
  those changes do not constitute a new instantiation of that
  message, and therefore the message would not get a new message
  identifier.  For example, when messages are introduced into the
  transport system, they are often prepended with additional header
  fields such as trace fields (described in section 3.6.7) and
  resent fields (described in section 3.6.6).  The addition of such
  header fields does not change the identity of the message and
  therefore the original "Message-ID:" field is retained.  In all
  cases, it is the meaning that the sender of the message wishes to
  convey (i.e., whether this is the same message or a different
  message) that determines whether or not the "Message-ID:" field
  changes, not any particular syntactic difference that appears (or
  does not appear) in the message.

  (RFC 5322)

I don't see the bit about "discard".

If your MUA were to discard any message it saw twice, you'd never
receive a bounce, and would be mystified about what was happening
to those messages. If your Mail Submission Agent or any Mail
Transport Agent along the way discarded "duplicates", how would
you send the same message to more than one recipient? Ditto for
your own MTA receiving messages for users on your system.

Cheers,
David.