错误表现为能发不能收，同域下也不行。

日志：

Sep 10 09:05:24 mail postfix/qmgr[9788]: C4351101654: to=<nq@networkquestions.org>, relay=none, delay=0.33, delays=0.25/0.08/0/0, dsn=4.3.0, status=deferred (unknown mail transport error)
解决方法：编辑/etc/postfix/master.cf，屏蔽红色部分。

# maildrop. See the Postfix MAILDROP_README file for details.
# Also specify in main.cf: maildrop_destination_recipient_limit=1
#
maildrop  unix  –       n       n       –       –       pipe
#  flags=DRhu user=vmail argv=/usr/local/bin/maildrop -d ${recipient}
  flags=DRhu user=vuser argv=maildrop -w 90 -d ${user}@${nexthop} ${recipient} ${user} ${extension} {nexthop}
#

重启postfix