Thanks for the reply.
When I go to
mail.mydomain.com in Chrome, I first get the warning about it being insecure (because of the lack of SSL). If I click through and tell Chrome "go there anyway," I get this message:
mail.mydomain.com normally uses encryption to protect your information. When Chrome tried to connect to mail.mydomain.com this time, the website sent back unusual and incorrect credentials. This may happen when an attacker is trying to pretend to be mail.mydomain.com or a Wi-Fi sign-in screen has interrupted the connection. Your information is still secure because Chrome stopped the connection before any data was exchanged.
You cannot visit mail.mydomain.com right now because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.
I doubt it will work later, because my web host's automatic SSL certificate renewal process failed with
mail.mydomain.com as I explained in my original post.
Anyway, I have no need for
mail.mydomain.com, so I'll just turn it off in FM's DNS settings.