@classmethod
def update(cls, domain, source, dest_add, dest_del):
    """Update a domain mail forward destinations."""
    result = None
    if dest_add or dest_del:
        current_destinations = cls.get_destinations(domain, source)
        fwds = current_destinations[:]
        if dest_add:
            for dest in dest_add:
                if dest not in fwds:
                    fwds.append(dest)
        if dest_del:
            for dest in dest_del:
                if dest in fwds:
                    fwds.remove(dest)
        if len(current_destinations) != len(fwds
            ) or current_destinations != fwds:
            cls.echo('Updating mail forward %s@%s' % (source, domain))
            options = {'destinations': fwds}
            result = cls.call('domain.forward.update', domain, source, options)
    return result
