Which Secure Delivery Mode is not an option with Mimecast?

In the context of Mimecast's Secure Delivery Modes, it is essential to understand the various options available for the secure transmission of emails. The statement that "No TLS - Fall Back to Secure Messaging" is not a valid option signifies a key aspect of how Mimecast prioritizes email security.

Mimecast offers modes that emphasize the use of Transport Layer Security (TLS) to ensure encrypted communications. This stems from the necessity to maintain confidentiality and integrity when emails are in transit. The options that include TLS (such as "TLS Required," "Only TLS," and "Forced TLS") focus on ensuring that emails are encrypted, which is vital in today's cybersecurity landscape.

However, the option "No TLS - Fall Back to Secure Messaging" implies a lack of a secure protocol as a default and suggests a fallback mechanism instead of enforcing security from the outset. In practice, moving away from TLS reduces the overall security posture of email communications, which is contrary to Mimecast's goal to provide robust protection against threats such as data breaches and phishing attacks. Hence, not offering this option aligns with best practices in email security, emphasizing the importance of using TLS as a standard for secure email delivery.