Paul Ssekamatte via Mailman-users writes:

 > 1. strangely so far i only see Yahoo domain in the spam
 > 2. I use Postfix as the MTA
 > 3.  The MTA only relays for a specific list of senders
 > 4. It is a production mail server so we have most of those controls in place 
 > i.e DKIM, SPF 
 > 5. I have not identified any suspicious logins, the bad actors must be using 
 > the subscription option on my mailing lists info page

OK, I would say the chances are that Mark is right.
There is another channel that could cause the same effect, which is
mail subscriptions to $LIST-join@$YOUR.DOMAIN (also $LIST-subscribe
and $LIST-request variants on many systems).

Steve

-- 
GNU Mailman consultant (installation, migration, customization)
Sirius Open Source    https://www.siriusopensource.com/
Software systems consulting in Europe, North America, and Japan
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