I use FastMail as well. I actually went from FastMail to Proton, and then back to FastMail.

I configure a lot of my home networking equipment and servers to send me emails for things like installing software updates, reboots, and any possible security issues. I couldn’t make that work with Proton not supporting any standard mail protocols (I know about Bridge, but it requires a GUI, and I didn’t want to use a third party program).