Proton. There are some other good alternatives. But since the rest of the family was also using Fastmail, I needed a solution that was user-friendly enough. Besides that, Proton Drive also made it possible to finally move away from our Dropbox Family subscription.
Downside is that their main servers are in the US, which may be problematic these days if you are from outside the US.
I just moved away from Fastmail after 10+ years for this reason.
as of this week, I've been going through the rigmarole of self-hosting my own email (again), for the same reason as you
any avoidable dependency on the US has become a red line
don't forget to tell fastmail that the reason you're leaving is because they host in the US!
(I also told them if they open a DC outside the reach of the US regime: I be happy to become a customer again)
Out of curiosity: Where did you migrate to?
Proton. There are some other good alternatives. But since the rest of the family was also using Fastmail, I needed a solution that was user-friendly enough. Besides that, Proton Drive also made it possible to finally move away from our Dropbox Family subscription.