求助！！！！！ sent=Service unavailable

cargoboy

AIX 上有些邮件发不出去，mqueue下的log 信息如下所示：
Jun 20 11:28:22 brio36bcs02 sendmail[32046]: l5KFSG622346: to=<hoffmanj@126.com>,<jethomps@126.com>, delay=00:00:06, xdelay=00:00:00, mailer=relay, pri=20786154, relay=d01av01.pok.ibm.com. [9.56.224.215], dsn=5.0.0, stat=Service unavailable
Jun 21 10:10:25 brio36bcs02 sendmail[39286]: l5LEAI650418: to=<hoffmanj@sohu.com>,<jethomps@sohu.com>, delay=00:00:07, xdelay=00:00:00, mailer=relay, pri=20856425, relay=d01av02.pok.ibm.com. [9.56.224.216], dsn=5.0.0, stat=Service unavailable
Jun 23 08:52:48 brio36bcs02 sendmail[38560]: l5NCqg644834: to=<hoffmanj@126.com>,<jethomps@126.com>, delay=00:00:06, xdelay=00:00:00, mailer=relay, pri=20969962, relay=d03av01.boulder.ibm.com. [9.17.195.167], dsn=5.0.0, stat=Service unavailable

请高手指点，不胜感激！！！！！！！！！
发送成功的log如下:

Jun 25 07:00:10 brio36bcs02 sendmail[48734]: l5PB09638586: to=<cdang@us.ibm.com>, delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=121057, relay=d01av04.pok.ibm.com. [9.56.224.64], dsn=2.0.0, stat=Sent (l5PB0ApN006575 Message accepted for delivery)