Last I looked, Thunderbird used Exchange Web Services to connect to Office365 which Microsoft is getting rid of: https://techcommunity.microsoft.com/blog/exchan

Last I looked, Thunderbird used Exchange Web Services to connect to Office365 which Microsoft is getting rid of: https://techcommunity.microsoft.com/blog/exchange/retirement-of-exchange-web-services-in-exchange-online/3924440 https://techcommunity.microsoft.com/blog/exchange/retirement... (I point out Office365 since vast majority of "Exchange" users are on 365) It also only support email and not calendaring/contacts. That being said, Office365 Web Client is pretty good at this point and someone who doesn't live in Office all day can probably get along fine with it.