There is zero requirement to use any server or service associated with either New Vector or the Matrix Foundation.
There is also zero requirement to use the server (synapse, dendrite) or client (Element) implementations developed by these guys.
For example, I use conduwuit for my home server, with neochat and fluffychat clients, and they talk using published APIs and nothing else...
Larry is a civil servant, he stays.
ws2k12r2 is still getting updates, what w10? ;)
In the meantime, Evolution has had EWS support for years... no Rust involved.
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.
I've been using Voipo for years, no complaints.