I wish all the university email systems can send a "this email no longer exists" notice to anything sent to an deactivated email address. Since leaving my last job, I have missed quite a few emails from collaborators.
@commonchaffinch That's to throw a bunch of guesses at a mail server, and wait for the return of 'no address here'. A success is one that isn't returned. You should have put a 'forwarding' on the old email, if that was possible.
@hasmis Thanks for the explanation. Is it a form of cybersecurity attack? Alas I cannot set up the forwarding anymore for this one, but good to know for the future.