mail::Header::plain — An unstructured, US-ASCII-only, E-mail header.
        #include <libmail/headers.H>
        
        mail::Header::plain plain_header("Content-Type", "text/plain");
        
        std::cout << plain_header.toString();
        
This class creates an unstructured E-mail header. Two
      arguments are provided to the constructor: the header's name,
      and its contents. The header contents must contain only
      US-ASCII characters. Use the
      mail::Header::encoded(3x)
      object to create a header containing non-US-ASCII content.
The toString() returns the
      header as a single text string: “name: value”.
      Long headers are folded at the nearest whitespace.