It is utilizes Tor hidden services which have been
Allegedly broken by the NSA. Tor and I2P are not reliable anonymity. Do not rely on them. As well, they are probably honeypots so using them brings you to the attention of the NSA....
Fixed, The question was not what is uncrackable but "free & easy-to-use encryption software for email". The only thing I would trust is paper/pencil in a closet with a flashlight and a lighter to be NSA proof. And I'd sweat that as well.
But thank you for your correction.