Preface: Crimes that use computer networks or devices to advance other ends includes Phishing scams and Spam.

S/MIME technical background:
S/MIME is based on asymmetric cryptography to protect your emails from unwanted access. It also allows you to digitally sign your emails to verify you as the legitimate sender of the message, making it an effective weapon against many phishing attacks out there. That’s basically the gist of what S/MIME is all about.

Technical limitation:
Because S/⁠MIME takes into account interoperation in non-MIME environments, several different mechanisms are employed to carry the type information, and it becomes a bit difficult to identify S/⁠MIME messages.

Vulnerabilities:
Vulnerabilities found on Cisco Email Security Appliance

Impact: A remote attacker could exploit these vulnerabilities to cause a denial-of-service condition.

Cisco Email Security Appliance URL Filtering Denial of Service Vulnerability:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190109-esa-url-dos

Cisco Email Security Appliance Memory Corruption Denial of Service Vulnerability
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190109-esa-dos