Found this explanation:
https://mail.python.org/pipermail/mailman-users/2015-December/080211.html
It includes this:
Let us say you have 5 users on your list, user1 at aol.com, user2 at
aol.com, user3 at aol.com user4 at aol.com and user5 at aol.com. user1 at
aol.com posts a piece of mail to your list. mailman tries to deliver to
user2, user3, user4 and user5 @ aol.com aol says "drop dead, we don't talk
to you because of our DMARC policy" mail to user2 user3 user4 and user5
bounces. Their bounce count is incremented.
user2 at aol.com posts a piece of mail to your list. mailman tries to
deliver to user1, user3, user4 and user5 @ aol.com aol says "drop dead, we
don't talk to you because of our DMARC policy" mail to user1 user3 user4
and user5 bounces. Their bounce count is incremented. and so on. Every time
somebody from aol sends mail to the list, it bounces for every other aol
member on your list. Their bounce counts increase.
One day, some message sends some of the bounce counts over the limit
mailman has, after which it says -- Too many bounces! I cannot deliver mail
to this account! Unsubscribe this person! And, because of the way things
have happened you get a triggering message which causes a lot of
unsubscribes _from the same site_.
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software | http://www.hyperactivesw.com
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