[Bug 1845751] Re: cc modification due to nodup setting breaks DKIM & thus DMARC

[Eric Blake]

I have long argued that there are two orthogonal things, currently both
being managed by the single Avoid Duplicates settings.  One is good, one
is horrible.  I would really love to have TWO separate knobs so that
users can choose which of the two features they want.

1. Suppress duplicates - if a user does not want the same mail received
twice through two paths, the list should not mail that user when their
name is on a to or cc.  This feature is good.

2. Rewrite CC - when the list suppresses sending to a given user, it
rewrites mail headers to drop that user from cc, on the grounds that
anyone replying to the list will still get their reply to the original
author (through the list, rather than through cc).  This feature is bad.

I _really_ want a way to enable JUST feature 1, and NOT feature 2, for
the mailman lists I am subscribed to.

Here's several scenarios where I find feature 2 to be a misfeature,
where User A is a user with the feature enabled, users B and C are list
subscribers, and user D is not a list subscriber.

Scenario 1: User B writes to the list and to user A in cc (to catch user
A's attention).  User A gets a copy of the email where they are on CC,
but user B and user C get a copy of the email where user A is not on CC.
If user C replies to all, the reply will reach user A via the list, but
will NOT have user A in cc, failing to catch user A's attention on the
followup.

Scenario 2: User D writes to the list and to user A in cc (unaware that
user A is subscribed to the list, and trying to catch user A's
attention).  User A gets the copy of the email where they are on cc, but
user B and user C do not see that user A was in CC.  Furthermore, user B
replies all to the list and to user D, and now user D sees that user A
was dropped from the CC.  User D now wonders why user B dropped user A
from the conversation.

Avoiding the munging of CC when DMARC rules are involved is nice, but
does not go as far as I want, which is to have a knob for an individual
user to ALWAYS avoid the munging of their own email, regardless of
whether duplicate delivery is suppressed, and regardless of DMARC.

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https://bugs.launchpad.net/bugs/1845751

Title:
  cc modification due to nodup setting breaks DKIM & thus DMARC

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