Given that Fastmail is Australian, it is fundamentally untrustworthy in much the same way any US company is. Of course, Protonmail is pretty untrustworthy also. Most of that is down to them making claims that are patently untrue (the encryption keys are stored serverside, so their encryption is not better than symmetrical for one), but some of it is also just because they are not in a country with strong privacy guarantees (for that I tend to look at Scandinavia or Germany).