The paper (PDF) Email Security Issues, Tools, and Techniques Used in Investigation reviews real-world email forensic incidents and the tools used to mitigate threats.
For a technical look at how organizations try to secure their domains, security.txt Revisited: Analysis of Prevalence and Conformity examines the adoption of standard security reporting methods.
Research on Features Analysis of All Email Components for Phishing Detection discusses how attackers lure victims into providing the credentials that eventually end up in these "MiX" lists. Defensive Best Practices
Using protocols like PGP or S/MIME to ensure data remains unreadable if intercepted.