Turtle Mail

Turtle Mail is an easy-to-use mail server (SMTP/IMAP/POP3) which supports on-disk RSA-based encryption of e-mails, compatible with any client depending on the setup.
It also has support for redundancy which is also easy to configure and introduces two new protocols, TMCP and TMSP, "Turtle Mail Client/Server Protocol" which allows for better client-server and server-server communication.

The RSA keys used for mail encryption are stored on the server, although encrypted, but can either be decrypted by the server, if the client doesn't support it, or by the client, which is the most secure option.

Turtle Mail is currently under heavy development.