I have done some fairly extensive examination of logs and list settings for
one of the lists hosted here, and I can't come up with a sensible
explanation to tell a customer.
The list is set to not send messages when a person unsubscribes.
The bounce settings are set so that 2 warning messages are sent after the
address is disabled and those messages should be 7 days apart.
A message was sent to the list on 11/1. I see in the logs that a
substantial number of addresses reached the bounce limit and were disabled.
So, based on the bounce settings, many of these should have been
unsubscribed 14 days later. They were not.
However, on 11/24, there was a mass unsubscribe of addresses that had been
disabled. Additionally, many (including some that turned out to have valid
addresses) received unsubscribe notices.
I tend to think that these are not related, but are two issues that I have
not been able to explain.