What do you think of Bote mail in I2P, which uses DHT for relaying mail? It is set to hold mail in the DHT for 100 days or until fetched by the recipient. It also uses public keys as addresses (ECDSA and NTRU are the options), and all mail is encrypted. I2P provides traffic anonymization, and Bote mail supports letting mails be relayed with random delays to further anonymize the sender by removing time correlation.
I would love to see someone give i2p-bote a good analysis. I tried using it a couple of times but wasn't able to ever receive messages successfully. They are advertixing some pretty awesome features though; no content or metadata leakage; it is secure against a global passive adeversary if you use delays between relay hops; parties don't have to be online at the same time to communicate.
I've never had problems with receiving messages with it. Had I2P been running for at least 20 minutes or so to be able to establish enough connections? Bote can tell you how many Bote nodes it is connected to.
https://www.i2p2.de and http://i2pbote.i2p.us (remove .us if you have I2P installed).