[Mailman-Users] Many users unsubscribed at once (not by me)

[Hal]

Today I received 18 unsubscribe notification mesages, each one telling 
me that "name at addr.ess has been removed from my_list_name".
They all appear to have been sent simultaneously, and since I (the list 
adminisistrator) haven't performed the unsubscribe request myself I 
suspect either some sort of automated request by the Mailman software or 
someone hacking into the system.
I've had this happen to me once before, several months ago.

The only thing I can think of apart from someone hacking into the 
list-server is that Mailman automatically removes members who have 
non-working addresses after a certain amount of list postings. Could 
this be it, or am I right to be worried and should contact the server 
owner to ask if an intrusion has shown up on the logs?
Logging into the Mailman admin page for my list tells me I'm using 
Mailman 2.1.12.

Hal