Email security and encryption are essential for reliable email sending. There are two major security protocols used for email delivery: SSL and TLS.

There are three encryption requirement options available to use with Start TLS:

1. No TLS
2. Opportunistic TLS
3. Enforced TLS: 

The Dynamics 365 Customer Insights - Journeys sending infrastructure uses the most flexible and reliable option, opportunistic TLS.

Customers send sensitive personal data (information about membership in a trade union) and they are not allowed to send without Forced TLS as a minimum.

Could you please enable Enforced TLS?