At SCRYPTmail, we've done extensive research and development on features that not only encrypt email messages but hide metadata as well. We call the process used to obfuscate the sender and receivers of email messages the ‘Seed To E-Mail’ technique.
When each email is encrypted using the public key of the intended recipient, a tiny message (or 'seed') is also created. This seed is created using the recipients second public key (or 'seedKey'). Both of these messages (the 'E-mail' and the 'seed') are sent to the SCRYPTmail server; however, neither of the messages have any metadata and are only seen by the server as encrypted “mumbo-jumbo”.
The ‘seed’ is really small and is simply a reference to the location of the E-mail. Consequently, pulling seeds and decrypting them is not an unreasonable task. If a seed is successfully decrypted, it indicates that a reference to a message is inside. The recipient then requests the message from the SCRYPTmail server and will be able to decrypt it as well.
This technique completely hides sender and recipient messages even with access to SCRYPTmail's servers.