MTA-STS is a mechanism that instructs an SMTP server that the communication with the other SMTP server MUST be encrypted and that the domain name on the certificate should match the domain in the policy.
It uses a combination of DNS and HTTPS to publish a policy that tells the sending party what to do when an encrypted channel cannot be negotiated.
When using Spam Experts services, steps and instructions for configuring MTA records for different mail servers at Configure Identity Header Identification Method in Spam Experts and Your MTA
