Thanks very much for your reply. No log file regarding amavisd-new.
but there is a sentence regarding amvisd-new in maillog:
May 17 21:52:15 xxx postfix/error[1332]: EC5352DE97:to=<xxx@xxx.xxx.xxx.xxx>, relay=none, delay=4690, delays=4690/0.07/0/0, dsn=4.4.1,
status=deferred (delivery...