​By this time, [log in to unmask] has already been served out (either by Listserv postmaster action, or more probably, by​ by that address sending Listserv email that Listserv did not understand).  It will stay "served out" until someone (anyone) sends Listserv a command "serve [log in to unmask]".

Now, if the mail that address is sending to your list is problematic (perhaps it is too big or contains nothing but HTML on a non-HTML list or whatever), Listserv may send an email back to it explaining the problem.  If  that email is not expected, perhaps it shoots something back to Listserv ... and the counter goes up ... hit a number (normally 50, as I recall) and Listserv gets tired of listening and serves the address out.