The mail::Header family of
      classes provide a convenient way for creating E-mail headers.
      The usual approach is to instantiate a mail::Header::list(3x)
      object, then use the “<<” operator repeatedly
      with any combination of mail::Header::addresslist(3x),
      mail::Header::encoded(3x),
      mail::Header::mime(3x) and
      mail::Header::plain(3x)
      objects, which format different kinds of E-mail headers.
      Finally, mail::Header::list(3x)
      std::string converts the headers
      to text, and returns a single string containing
      nicely-formatted headers.