From the website:

No central server = No third party to trust

Secure, 100% decentralized email that ONLY YOU can access!

There is no central point that stores your messages, so there is nowhere to steal or even submit a request for your private data. Even we cannot locate your messages or derive your password, ever. ONLY those who know your password have access.

Nxt does for email what Bitcoin has done for transactions; it makes them secure by placing the data they are made up of outside the control of any one central actor.