Creating headers

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.