I am part of a listserve Ecolog-L which has twice unsubscribed me for "repeated delivery error reports from your mail system.... while you are obviously able to receive mail, your mail system has been regularly reporting that your account did not exist, or that you were otherwise permanently unable to receive mail." I've also been warned this is not localized to the listserve, but could also have affected other personal emails as well. I have no idea what to do about this.