If you have your own domain, Fastmail banning you is a minor speed bump at worst. You can just point your domain at a different email provider.
You can similarly transfer domains between registrars if one of them doesn't like you. And in practice, your domain registrar not liking you is pretty rare; I've only heard of it happening with famously evil alt-right websites and they generally manage to find a registrar that will take their business.
> You can similarly transfer domains between registrars if one of them doesn't like you.
That's sadly not true.
There are various examples of individuals loosing their domains as registrar's refuse to release them. The transfer only works if both registers play ball, so it's essentially a very similar situation.
