https://z-lib.gs/

https://hackandpwn.com/windows-8.1-and-2012-r2-esu-analysis/

ws2k12r2 is still getting updates, what w10? ;)

Radicale. Not the fastest, but has worked for years.

I use Evolution with the o365 account at work. All one needs is the EWS plugin and the company tenant ID. Evolution then pretends to be a generic MS Office app, using a well-known app ID. oauth2 works just fine.

Giving customers more control? Great.