Why are there bounces for users who aren't subscribed?
Most often, this is due to subscribers whose accounts forward mail someplace else and that final destination is broken. This delivery failure generates a bounce that Ecartis sees as coming from someone who isn't on the list, since, after all, that address isn't.
Another common cause is SPAM being sent to the -bounce address. Whatever the SPAMmer decides to drop in as a “From” address becomes listed as bouncing.
These “notsub” bounces can be safely ignored.