code: def update(cls, domain, source, dest_add, dest_del):
          
          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
docs: Update a domain mail forward destinations.