Of course it is opt in. Why would it not be? Microsoft have opted in automatically on your behalf. Soon you will only be able to opt in, for your convenience, as too many people were accidentally opting out. /s

That’s what happens when you make it expensive to import anything and don’t have any domestic manufacturing for computer components.

https://proton.me/ Are worth a look at. The allow custom domains and I believe have IMAP support. Additionally they encrypt everything they store so are very good from a privacy side (at least as far as you can be private using email).