List Spam Filtering

Rich Kulawiec rsk at
Sun May 12 12:22:54 UTC 2013

1. Restricting mailing lists to "subscribers only" has been a best
practice since the last century.  It's a very good anti-spam tactic.

2. However, doing so -- for a list run via Mailman, like this one --
does not pose a significant impediment for non-subscribers.  By default,
Mailman will hold traffic from non-subscribers for list-owner approval.
Provided the list-owners check that queue periodically and have reasonable
spam-spotting abilities, this works beautifully.

3. Note that Mailman, as part of that same mechanism, allows list-owners
to add non-subscribers to a list of those permitted to send traffic to
the list without approval.  This feature is probably more often used to
allow traffic from alternative addresses for subscribers, e.g., someone
is subscribed as fred at but sends occasionally from fred at
But it can just as easily be used for non-subscribers if the list-owners
so choose.

4. List-owners may also find it useful to keep track of which spammers
repeatedly attempt to abuse the list and block them at the MTA -- which
has the desirable side effect of blocking them from ALL lists.  I do this
on a user/host/domain/network basis, and it's proven itself to be worth
the effort.

So: setting the "subscribers-only" flag on Mailman has major advantages,
at the cost of additional work on the part of list-owners -- which can
be mitigated in part across all lists by making changes to the MTA.


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