Are you able to log in to the email account via the web interface (Roundcube)? Able to send email from there?
Looking at the error message further it seems to be complaining about TLS version. MIAB now publishes a policy over https but it also still supports old TLS versions.
What is the mail user agent? Outlook perhaps?
Is MIAB your DNS server or is that external? Have a look at some of the forum posts on ‘MTA STS’ policy for some other considerations.
This post might be useful: [SOLVED] V0.50 MTA-STS policy is missing: STSFetchResult.NONE