If you want to lessen your dependency on Microsoft, or other big technology companies, and went some ideas, here's what I do for to run my own small law firm on Free software:
https://decoded.legal/blog/2026/11/running-a-law-firm-on-free-software-2026-edition/
And, while I prefer to self-host, that is not a requirement - lots of small hosting companies could help with the hosted aspects of this.
@neil "I need to move our mail infrastructure at some point, and I want to look at Stalwart." I'm in the same boat, but when I looked at stalwart (0.16), I didn't really like it. Two things: one was that the non-paid version had annoying buttons in the web interface that users who log in to change their password would see (I forget the exact wording, but sort of "upgrade to Pro to unlock this feature!" kind of thing). The second was that so much happens under the hood that it's hard to debug.