Not disastrous but mildly annoying.
When I select my main email account it gives me a pink bar across the top saying that the account settings are wrong, with the choice of Fix It or Dismiss. If I click on Fix it, a box comes up with the email address already filled in, asking me to enter my password, which I do, and then the box goes away as does the pink bar. Nothing else happens. All my emails are already downloaded and showing on screen. Whatever I do, Fix it or Dismiss, the emails are there and it all works fine. It doesn't ask for any other settings, and I've checked them all and they are OK.
I have 3 email addresses. The main one is a Smarter Mail address, and I used to access it through my browser (Safari, Chrome, Edge whatever) with no problems. Got a new laptop, started using Mail a few months ago. After a month or so it started doing this ^^ whenever I had my main email account selected.
Not a massive headache, but annoying.