def get_prep_value(self, value):
        """
        We need to accomodate queries where a single email,
        or list of email addresses is supplied as arguments. For example:

        - Email.objects.filter(to='mail@example.com')
        - Email.objects.filter(to=['one@example.com', 'two@example.com'])
        """
        if isinstance(value, six.string_types):
            return value
        else:
            return ', '.join(map(lambda s: s.strip(), value))