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.
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.