Attach an email header to send with the message.

        :param name: The name of the header value.
        :param value: The header value.

def add_header(self, name, value):
        '''Attach an email header to send with the message.

        :param name: The name of the header value.
        :param value: The header value.
        '''
        if self.headers is None:
            self.headers = []
        self.headers.append(dict(Name=name, Value=value))