These requests originated from over 200 IP addresses – almost all owned by an ISP for a particular province in China. The confirmation emails for this volume of requests overwhelmed our email service. As a result, many arXiv users may not have received their daily emails. And other users may not have received their confirmation emails for registering accounts, or legitimate email change requests. this should be easy…
The email requests already happened. So blacklisting the IP addresses wouldn't prevent the email overload.