Office 365 / "Modern Authentication" / Domain name not resolved

I’ve been using Em Client for a number of years with Office 365 on a corporate account. What Microsoft calls “work or school” if you login to the web page. When we enabled two factor authentication I used an “App Password” with Em Client.
Now this has stopped working, Microsoft has disabled simple password authentication.
According to various web pages, Em Client should support this, but I can’t get it to work at all.
I deleted my existing account, selected the option to add account, then after entering my email address it fails with “The domain name cannot be resolved”.
It doesn’t even get as far as checking credentials.
Any suggestions?