lookup users by mail address.
        you may do in two ways:
            - specify one address: foo@bar.com
            - specify a domain   : @bar.com
        both will return a list.

        the role 'Zeiterfasser' may not lookup users.

def by_mail(self, mail, archived=False, limit=None, page=None):
        """ lookup users by mail address.
        you may do in two ways:
            - specify one address: foo@bar.com
            - specify a domain   : @bar.com
        both will return a list.

        the role 'Zeiterfasser' may not lookup users.
        """
        path = partial(_path, self.adapter)
        if not archived:
            path = _path(self.adapter)
        else:
            path = _path(self.adapter, 'archived')
        return self._get(path, email=mail, limit=limit, page=page)