code: def prepare(self, mime):
          
          for key in self:
              if key == 'Bcc' or key == 'Resent-Bcc':
                  continue
              del mime[key]
              # Python 3.* email's compatibility layer will handle
              # unicode field values in proper way but Python 2
              # won't (it will encode not only additional field
              # values but also all header values)
              parsed_header, additional_fields = parse_header(
                  self[key] if IS_PY3 else
                  self[key].encode("utf-8")
              )
              mime.add_header(key, parsed_header, **additional_fields)
docs: Prepares a MIME object by applying the headers
          to the *mime* object. Ignores any Bcc or
          Resent-Bcc headers.