help me with step to set up sendmail with mailman

[somtob.kaewbowon@xxxxxxxxxxxxx]

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@localhost

   ----- The following addresses had permanent fatal errors -----
somt@xxxxxxxxxxxxx
    (reason: 550 5.1.1 <somt@xxxxxxxxxxxxx>... User unknown)
    (expanded from: somt@xxxxxxxxxxxxx)

   ----- Transcript of session follows -----
... while talking to [127.0.0.1]:
>>> DATA
<<< 550 5.1.1 <somt@xxxxxxxxxxxxx>... User unknown
550 5.1.1 somt@xxxxxxxxxxxxxxxx User unknown
<<< 503 5.0.0 Need RCPT (recipient)

thanks for help
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