Which of the following is NOT a supported Secure Delivery Mode with Mimecast?

The correct answer highlights a mode of operation that is not recognized within Mimecast's Secure Delivery Modes. Default/Opportunistic mode typically refers to an automatic selection process where a secure connection is used when available, but if it is not, the system may fall back to an unencrypted method. While this could be part of general email transmission procedures, it does not specifically identify a distinct secure delivery mode supported by Mimecast.

On the other hand, the other modes mentioned, such as No TLS - Fall Back to Secure Messaging, Enforced TLS, and X-Enforced TLS - Fall Back to Secure Messaging, are indeed established and recognized configurations within the Mimecast environment.

Enforced TLS means that the system requires the use of Transport Layer Security for email transmission, ensuring that messages cannot be sent unencrypted. X-Enforced TLS - Fall Back to Secure Messaging also mandates TLS for protection but allows a fallback mechanism, ensuring messages can still be sent securely even if standard TLS cannot be established. These established configurations provide different levels of security for email delivery, emphasizing Mimecast's commitment to safeguarding message integrity and confidentiality.

Therefore, identifying Default/Opportunistic as not a supported mode aligns with the recognized delivery modes designed to enhance email security while providing operational