message-required-mail-headers

message-required-mail-headers is a variable defined in `message.el'.
Its value is

(From Subject Date
      (optional . In-Reply-To)
      Message-ID
      (optional . User-Agent))

Documentation:
*Headers to be generated or prompted for when mailing a message.
It is recommended that From, Date, To, Subject and Message-ID be
included. Organization and User-Agent are optional.

You can customize this variable.