Letter

Letters represent the core body of a mail, comprised of the data being transferred and classifications on what to do with the data. Letters are assembled by the mail sender.

pub struct Letter {
    is_response_expected: u8, // 0 false, else true 
    is_executable: u8, // 0 false, else true
    x_data: Binary, 
}

Key	Type	Description
is_response_expected	u8	If set as true, receiver should send back a response mail, otherwise mail marked as failed.
is_executable	u8	If set as true, letter contents should be executed post-receival, otherwise mail marked as failed.
x_data	Binary	Base64-encoded JSON of message.