[Mailman-Users] error: user unknown

[=?gb2312?B?uN/JvQ==?=]

Hi,
Just now, I installed Mailman2.1.4 on my computer running Fedora Core 1.
I followed INSTALL and created a new list named "ccc" for test.

Finally I got this error message.
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Hi. This is the deliver program at mails.tsinghua.edu.cn.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

ccc at tu069007.tsinghua.edu.cn <-------------this is my computer
550 5.1.1 ... User unknown 
--- Attachment is a copy of the message.
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It told me there is not a user called ccc(mail list's name)
I use Sendmail and Mailman can send me mail.
How to solve this problem?
Thanks.

gaoshan