:param to: Email addresses for the 'To' API field.
        :type to: :keyword:`list` or `str`

def to(self, to):
        '''
        :param to: Email addresses for the 'To' API field.
        :type to: :keyword:`list` or `str`
        '''
        if isinstance(to, basestring):
            to = to.split(',')
        self._to = to