STARTTLS offered
smtp.starttls
Without STARTTLS, mail crosses the internet in plain text and anyone on the path can read it.
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
STARTTLS offered is one of 49 checks in every report, alongside delegation, mail authentication, TLS and registration.