ISC2 Exam SSCP Topic 10 Question 55 Discussion

Content security measures presumes that the content is available in cleartext on the central mail server.

Encrypted emails have to be decrypted before it can be filtered (e.g. to detect viruses), so you need the decryption key on the central 'crypto mail server'.

There are several ways for such key management, e.g. by message or key recovery methods. However, that would certainly require further processing in order to achieve such goal.