Using Classic Outlook on Windows for personal email, calendar, and contact management
I have fallen back on the Office 16 version of Outlook. It works reliably for me with my existing btinternet e-mail address. I could not find any way to add this to the 365 integrated version: no tab or three dots etc. I read elsewhere that Microsoft are now only allowing gmail additions to the 365 Outlook, but Bing claims btinternet should work. My gmail address certainly works on the 365 version.
Only outlook.com / Microsoft 365 business and Gmail addresses are currently supported in the new outlook. It also doesn't support offline use.