MTA-STS
mailauth.mta-sts
MTA-STS stops an attacker on the path from stripping STARTTLS and reading your mail.
A longer write-up of this check has not been published yet. The report itself shows the raw evidence behind every result, which is usually the fastest way to see what was measured.
Run this check on a domain
MTA-STS is one of 49 checks in every report, alongside delegation, mail authentication, TLS and registration.