Why do you need MTA-STS?
The Simple Mail Transfer Protocol (SMTP) alone does not provide security, making it vulnerable to malicious attacks such as man-in-the-middle attacks. A man-in-the-middle is an attack where communication between two servers is intercepted and possibly changed without detection by the recipient.
In addition, encryption is optional in SMTP, which means that emails can be sent in plaintext. If a plaintext email was intercepted in transit, it could easily be read and manipulated. Without MTA-STS, an attacker can intercept the communication and force the sending service to send the message in plain text. By enabling MTA-STS, a TLS connection is required which ensures encryption and keeps your emails private.