Exam CAS-004 topic 1 question 643 discussion

The most likely cause of the issue is D. The external recipient's email address and the email address associated with the external recipient's public key are mismatched. When sending encrypted emails, especially using protocols like S/MIME (Secure/Multipurpose Internet Mail Extensions), the recipient’s public key is used to encrypt the message. This public key is typically associated with the recipient's email address. If the email address in the recipient’s public key does not match the actual email address of the recipient (for example, if the email address is outdated or incorrect in the public key directory), the internal user's email client may not be able to encrypt the email correctly, leading to an error. Since the internal user can send unencrypted emails successfully, this indicates that basic email delivery is not an issue. The problem specifically arises when encryption is involved, suggesting that it is tied to the public key or email address mismatch rather than a server or broader system configuration problem.