Think I figured it out.
For anyone running into this...
Backup your settings.json in your domain folder before starting this... just in case.
In the domains settings.json file delete everything between the square brackets following "acme_certs":
Make sure this ends up reading
.... "acme_certs":[], ...
where the ... before and after is lots of more json coding.
Be careful you are only removing what is part of that square bracket encompassing region.
SAVE the config
[that little step I found in another post here in the Community]
Then in the web admin interface I disabled the auto certs setting
SAVE
Made sure all the certs for smartermail were removed from IIS Cert Manager
Not sure if necessary, but I restarted the IIS service at this point.
Not sure if it was necessary but I restarted the SmarterMail service at this point.
Now went back into the web interface and renabled the AutoCerts feature.
It automatically regenerated the IIS certs and the certs in the listing of certificates in SmarterMail... and it created a new AutoCert for the mail.mydomain.com. But, the same thing that caused this whole situation to begin with still exists. There is a 2nd AutoCert for
mail.mydomain.com in the AutoCerts listing that I can't find anywhere to remove it. I have now just disabled it and will leave it at that. Annoying but at least functional.