regarding IETF lists using mailman: nodupes considered harmful

I'm sending this to the ietf list because mailman is widely used for
IETF mailing lists.  

Recent versions of mailman appear to have a flaw that allows the sender
of a message to send a copy to everyone in a mailing list _except_ some
set of explicitly specified recipients, and there will be no indication
that those recipients were excluded.  In the context of an IETF mailing
list this can be used to exclude participation by certain individuals.

Specifics:  Mailman has a per-recipient setting called "nodupes".  The
effect of this setting is that any recipient address that has the
nodupes flag set, and which appears in a To or CC field of a message
sent to the list, will not receive a copy of the message from the
list.  It also appears (from emperical testing with a limited sample)
that the list may also remove that recipient's address from the CC
field of the copy of the message sent to the list.

Recent versions of mailman set "nodupes" by default.  List participants
can change it if they know about it, but they might have no idea of how
this setting works and how it can be used to exclude them from
participation.

The workaround is to set "new_member_options" (under General Options)
so that "Filter out duplicate messages to list members" is NOT set.
"Do not send a copy of a member's own post" should probably not
be set either.

Keith

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