There seems to be a big problem with this the last 2 weeks.
2 Weeks ago a sponsor I emailed one time from my own servers e-mail started getting over 10000 e-mails a day. A few days later It started happening to my server and I eventually had to delete the e-mail account and clean the server.
This seems to be a Russian virus that's spread through e-mail.
Blocking the sender doesn't work but if you're able to bounce the e-mail back that may work.
Unless you have your own server I don't think you can do anything except delete your e-mail account.