Isn't fastmail an Australian company? If so then they'll have to backdoor all software in the future since parliament passed these insane new surveillance laws.
Email is plain text anyway so why would it matter? If you want to keep it private you’ll have to encrypt it anyway and then all their backdoors wouldn’t change that.
There is a great difference between in-transit encryption and at-rest encryption. The former is not really comparable, as the existence of the email at rest will forever be vulnerable, in contrast to the time of transmission. A window of seconds versus years.
The backdoor problem is a serious issue, but TCNs aren't necessary for email providers. All your emails are already I'm plaintext.
I'd be more worried about the fact that TARs and TANs have no judicial overview and are secret in a way that other kinds of warrants aren't. But then again, you aren't the one being served with a warrant.
As an Australian, I wouldn't use it anyway. Mailbox.org is better IMHO, and is what I use.