[Mailman-Users] DMARC issues

Mark Sapiro mark at msapiro.net
Sat Apr 12 03:53:09 CEST 2014

On 04/11/2014 06:28 PM, Peter Shute wrote:
> I hadn't heard of this till now. Could somebody please confirm if my understanding of the issue is correct?
> This is what I'm thinking will happen, please correct where I'm wrong:
> - A list member sends an email to the list from a yahoo address
> - The list sends that email out to all the list members
> - The recipients' mail servers will (might?) check with yahoo what to do with the email, and will be advised to reject it
> - The list will receive a bounce for every email address whose mail server follows that advice
> - Those recipients whose mail server follows the advice will not receive the message
> - The list will increment the bounce score for all those affected receipients, but only once per day
> - The increment will be 1 because this is a hard bounce
> - If the score reaches the bounce_score_threshold before the bounce_info_stale_after number of days has passed since the most recent bounce, then the member's subscription is disabled.


> If that's correct then my understanding is that:
> - If a list has at least one active yahoo member then pretty soon everyone's subscription will be disabled (not unsubscribed?).

Everyone whose ISP honors Yahoo's DMARC reject policy. And they will
eventually be unsubscribed after (bounce_you_are_disabled_warnings) *
(bounce_you_are_disabled_warnings_interval) days.

> - If a list receives vey few messages from yahoo addresses then the only effect will be that their messages don't get through, and that they might still get through to some people.

Maybe. Yahoo requests and receives reports of rejected mail. This is
only speculation, but if Yahoo sees that your server is sending what it
considers to be bogus mail purporting to be From: its domain, it could
decide to reject all mail from your server.

> I'm a moderator for a cpanel list, but don't have access to any of the settings. Can someone tell me what the default settings are for bounce_score_threshold  and bounce_info_stale_after? I'm assuming ours might still be whatever the defaults are.

The list admin can see these values on the list's web admin Bounce
processing page, but defaults are:

bounce_score_threshold = 5.0
bounce_info_stale_after = 7
bounce_you_are_disabled_warnings = 3 = 7	

> Am I right in thinking that if we make these values high enough, we'll see no accounts disabled, and the only side effects will be more bounces and yahoo mail won't get through? Would this be an acceptable solution for a list with only 1000 members and low traffic, assuming we warn the yahoo members to use a different address?

Just turn off bounce processing for the list. See the FAQ at

Also consider what I speculate above in the paragraph starting with "Maybe."

Additional reading at <http://www.dmarc.org/faq.html#s_3>,
<http://blog.threadable.com/how-threadable-solved-the-dmarc-problem> and
and other articles linked from those.

Mark Sapiro <mark at msapiro.net>        The highway is for gamblers,
San Francisco Bay Area, California    better use your sense - B. Dylan

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