Struct HeadersByConfidentiality 

struct HeadersByConfidentiality {
    unprotected_headers: Vec<(&'static str, HeaderType<'static>)>,
    hidden_headers: Vec<(&'static str, HeaderType<'static>)>,
    protected_headers: Vec<(&'static str, HeaderType<'static>)>,
}

Fields

unprotected_headers: Vec<(&'static str, HeaderType<'static>)>

Headers that must go into IMF header section.

These are standard headers such as Date, In-Reply-To, References, which cannot be placed anywhere else according to the standard. Placing headers here also allows them to be fetched individually over IMAP without downloading the message body. This is why Chat-Version is placed here.

hidden_headers: Vec<(&'static str, HeaderType<'static>)>

Headers that MUST NOT (only) go into IMF header section:

• Large headers which may hit the header section size limit on the server, such as Chat-User-Avatar with a base64-encoded image inside.
• Headers duplicated here that servers mess up with in the IMF header section, like Message-ID.
• Nonstandard headers that should be DKIM-protected because e.g. OpenDKIM only signs known headers.

The header should be hidden from MTA by moving it either into protected part in case of encrypted mails or unprotected MIME preamble in case of unencrypted mails.

protected_headers: Vec<(&'static str, HeaderType<'static>)>

Opportunistically protected headers.

These headers are placed into encrypted part if the message is encrypted. Place headers which are not needed before decryption (e.g. Chat-Group-Name) or are not interesting if the message cannot be decrypted (e.g. Chat-Disposition-Notification-To) here.

If the message is not encrypted, these headers are placed into IMF header section, so make sure that the message will be encrypted if you place any sensitive information here.