For people who are interested in such technologies, I recommend looking at Pond:
It is written by Adam Langley, so it comes with some serious credentials behind it. It provides asynchronous email-like messaging that's forward secure, resistant to traffic analysis and the whole thing runs over Tor. Messages are stored for a week and are strictly limited in size. There's no spam because nobody has an address - instead you have to grant someone the ability to message you by giving them a small file. So, not really intended as an email competitor convenience wise, but it has many interesting ideas and a reasonable GUI.
As a testament to the seriousness with which Pond takes forward security, it can use the NVRAM in a TPM chip to reliably destroy keys for data that an SSD device might have otherwise made un-erasable.
The main downside - it's written in Go :)