It is actually "fastmail", rather than "fashost", but that is scarcely relevant once one uses the IP address. Noted you had inserted the IP address in the relevant location, and so decided I would go to the Interface I have and generate a new AppPassword. I did so, and put it in the "Password" field. I then went to a record and tried to send the "Validate" email.
Please find in the private error the complete error record it generated, which I have also added to the open ticket over at the service provider, who have yet to respond to me.
Thank you for your kindness,