[Mailman-Users] DMARC issues

Barry Warsaw barry at list.org
Thu Apr 10 17:36:16 CEST 2014

On Apr 10, 2014, at 03:09 PM, Siniša Burina wrote:

>I believe there's no need to elaborate on the problems recently introduced by
>Yahoo, changing their DMARC DNS record and rendering many mailman lists
>unusable for Yahoo mail users.

It *is* a shame that these anti-spam defenses knowingly break mailing lists.
I say "knowingly" but not "maliciously" because their specs usually describe
the adverse affects on mailing lists, along with some mitigation approaches
(which may not have a positive effect on list usability <wink>).  The spec
authors are not hostile to mailing lists and would rather not break them, but
there does seem to be a fundamental conflict between mailing lists and
anti-spam approaches.

That said, DMARC was discussed in great detail last year on the -developers
list, so if you want all the gory details, check out those archives.

Mark will probably follow up in more detail, but MM2.1 implemented a feature
in 2.1.16 called "from_is_list" which is a ternary option for addressing the
effects of DMARC.  It has to be enabled by the site admin, and then list
admins can opt-in.  It's disabled by default for backward compatibility
reasons.  From Defaults.py:

# The following is a three way setting.
# 0 -> Do not rewrite the From: or wrap the message.
# 1 -> Rewrite the From: header of posts replacing the posters address with
#      that of the list.  Also see REMOVE_DKIM_HEADERS above.
# 2 -> Do not modify the From: of the message, but wrap the message in an outer
#      message From the list address.

So as you can see, two approaches are available, From: rewriting or outer
message wrapping.  Both are suboptimal for usability, but it seems like we
have no other viable option.  This is not yet implemented in MM3 because I
don't really like having to do it.  We might have no choice though.


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