Email received with no Sender Domain or Sender

One of my clients received an email today with no Sender Domain, Sender or From in the headers. How can I block this kind of spam without blocking the IP (as an Outlook.com server could send legitimate email too)? There seems to be nothing else to blacklist, surely this can't be right. This is the info from Mail Delivery reports. Event: success success Sender User: -remote- Sender Domain: Sender: <> Sent Time: Sep 6, 2018 2:40:14 PM Sender Host: mail-bo1ind01hn0209.outbound.protection.outlook.com Sender IP: 104.47.xxx.xxx