-- DKIM info in mail headers.
-- a  :: Maybe Algorithm   -- TODO: (required), signing algorithm (example: `rsa-sha256`)
-- d  :: Maybe String      -- TODO: (required), Signing Domain Identifier (SDID) (example: `netlib.re`)
-- s  :: Maybe Selector    -- TODO: (required), selector (name of the DNS TXT entry for DKIM, such as `baguette` for `_baguette._dkim.netlib.re`)
-- c  :: Maybe Algorithm   -- TODO: (optional), canonicalization algorithm(s) for header and body (ex: "relaxed/simple")
-- q  :: Maybe String      -- TODO: (optional), default query method (example: `dns/txt`)
-- i  :: Maybe String      -- TODO: (optional), Agent or User Identifier (AUID) (in practice, an email address)
-- t  :: Maybe Time        -- TODO: (recommended), signature timestamp (time = number, such as `1117574938`)
-- x  :: Maybe Time        -- TODO: (recommended), expire time (time = number, such as `1117574938`)
-- l  :: Maybe Int         -- TODO: (optional), body length (such as `200`)
-- h  :: Maybe String      -- TODO: (required), header fields - list of those that have been signed
-- z  :: Maybe String      -- TODO: (optional), header fields - copy of selected header fields and values
-- bh :: Maybe CryptoHash  -- TODO: (required), body hash
-- b  :: Maybe Signature   -- TODO: (required), signature of headers and body