[CentOS] help me with step to set up sendmail with mailman

[somtob.kaewbowon at cbs.pea.co.th]

1. mailman can send message to sendmail
2.but sendmail can't send the message to mail list

The original message was received at Thu, 14 Oct 2010 09:41:53 +0700
from apache at localhost

   ----- The following addresses had permanent fatal errors -----
somt at cbs.pea.co.th
    (reason: 550 5.1.1 <somt at cbs.pea.co.th>... User unknown)
    (expanded from: somt at cbs.pea.co.th)

   ----- Transcript of session follows -----
... while talking to [127.0.0.1]:
>>> DATA
<<< 550 5.1.1 <somt at cbs.pea.co.th>... User unknown
550 5.1.1 somt at cbs.pea.co.th... User unknown
<<< 503 5.0.0 Need RCPT (recipient)

thanks for help