I will tell you for a FACT it is not ! The only safe communication is by phone. Phones require warrants to be listened in on, but anyone can look at emails with or without a judges order. Safe-mail may be safe from hackers, but rest assured if any real authority wants in there they will be.
Hushmail a few years back gave authorities access to emails regarding steroid deals. Even states on their site now that they will cooperate with authorities. Acquiring gear as we know is a risk in every way.