mail::Header::list — Create a list of headers.
        #include <libmail/headers.H>
        
        mail::Header::list headers;
        
            headers << mail::Header::encoded("Subject", "Hello", "iso-8859-1")
                    << mail::header::addresslist("To")
                                                ("John", "john@example.com")
                                                ("Tom", "tom@example.com")
                    << mail::Header::mime("Content-Type", "text/plain")
                                         ("charset", "iso-8859-1");
        
        std::cout << (std::string)headers;
        
This is a helper class for formatting a list of headers.
      Each header is specified, using the << operator, which can take any
      combination of mail::Header::addresslist(3x),
      mail::Header::encoded(3x),
      mail::Header::mime(3x), or
      mail::Header::plain(3x).
The << operator makes
        an internal copy of the header object. The original object
        may be destroyed after the operator returns.
The std::string operator
      formats all headers, and returns the set of E-mail headers as
      a single string. Long headers are folded accordingly.