There's always Bitmessage: http://dkn255hz262ypmii.onion/index.php?topic=125663.msg862881#msg862881 Since it's decentralized, it doesn't rely on one central point of failure, like Tormail's server. The network stores (encrypted) messages for 2 days, so as long as you start your client once a day, you get messages and you don't need to have it running all the time.