Which of the following is NOT a requirement for encrypting an email using PKI?

The correct answer is based on the understanding of Public Key Infrastructure (PKI) and its role in email encryption. Having unique email addresses is not a requirement for encrypting an email using PKI because the encryption process relies on the cryptographic keys linked to an individual's certificate rather than the uniqueness of the email address itself.

In PKI, each user has a digital certificate that confirms their identity and contains their public key. The encryption of an email message occurs when the sender uses the recipient's public key, which is provided via their digital certificate, to encrypt the message. The underlying mechanism is focused on the security of the keys and certificates involved—rather than the uniqueness of email addresses.

In contrast, knowing the PIN for a Common Access Card (CAC) is essential for accessing the private keys stored on the card for signing and decrypting messages. Trusting DoD certificate authorities is critical because it ensures that the certificates being used are legitimate and recognized within the system. Finally, being configured for certificate revocation checking is necessary to validate that a certificate has not been compromised or expired, which is vital for maintaining the integrity of the encryption process.

Therefore, the uniqueness of email addresses does not impact the encryption process itself in the context of PKI.