def create_address(email, display_name=None):
    """Create and return an address unlinked to any user.

        :param email: The text email address for the address being created.
        :type email: str
        :param display_name: The display name associated with the address.
        :type display_name: str
        :return: The newly created address object, with the given email
            address and display name, if given.
        :rtype: `IAddress`
        :raises ExistingAddressError: when the email address is already
            registered.
        """
