1. Take Security Measures
  2. Email Security
  3. Protect your email domain from spoofing
  4. Keep monitoring

Keep monitoring

Email security is an ongoing process. Using email security solutions can aid you with this task.

 
Depending on your business' needs, you can take a look at some of the solutions listed here: https://www.gartner.com/reviews/market/email-security
 
Open source solutions exist as well, such as for example: https://www.proxmox.com/en/proxmox-mail-gateway