But it’s an Australian company that is subjected to draconian encryption and surveillance laws, so if privacy is important maybe not the optimal choice. https://www.fastmail.com/blog/access-and-assistance-bill/
But agreed the TAA bill is pretty bad as that means the company receiving the notice cannot disclose the fact that they've been requested to assist.
As far as email goes, I wouldn't use it for any private conversation. There's better tools for that.