According to the FBI, Business Email Compromise (BEC) and Email Account Compromise (EAC) have caused $12 billion in losses since October 2013. These scams target businesses and individuals. In this research, Digital Shadows outlines the declining barriers to entry for this type of fraud: https://info.digitalshadows.com/BECResearchReport-Infographic.html
This infographic outlines 5 ways that cybercriminals gain access to emails without conducting a phishing campaign or network intrusion.