Not using Fastmail, but I'm working on this now. Bye, Google.
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.