我的mail服务器和263服务器

我的mail服务器mail.foo.pku.edu.cn(162.105.107.31)可以从263收信，但不能往263发信，别的mail服务器如：peoplemail是可以发的。也可以往pku.edu.cn上发信。这到底是为什么？
＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋   **********************************************
    **      THIS IS A WARNING MESSAGE ONLY      **
    **  YOU DO NOT NEED TO RESEND YOUR MESSAGE  **
    **********************************************

The original message was received at Tue, 25 Sep 2001 15:56:49 +0800
from phytest.foo.edu.cn [162.105.107.18]

   ----- The following addresses had transient non-fatal errors -----
<luxr@263.net>

   ----- Transcript of session follows -----
.... while talking to mx03.263.net.:
>>> MAIL From:<lxr@foo.edu.cn> SIZE=649
<<< 450 <lxr@foo.pku.edu.cn>: Sender address rejected: Domain not found
.... while talking to mx02.263.net.:
>>> MAIL From:<lxr@foo.pku.edu.cn> SIZE=649
<<< 450 <lxr@foo.pku.edu.cn>: Sender address rejected: Domain not found
.... while talking to mx01.263.net.:
>>> MAIL From:<lxr@foo.pku.edu.cn> SIZE=649
<<< 450 <lxr@foo.pku.edu.cn>: Sender address rejected: Domain not found
<luxr@263.net>... Deferred: 450 <lxr@foo.pku.edu.cn>: Sender address rejected: Domain not found
Warning: message still undelivered after 4 hours
Will keep trying until message is 5 days old

Reporting-MTA: dns; mail.foo.pku.edu.cn
Arrival-Date: Tue, 25 Sep 2001 15:56:49 +0800

Final-Recipient: RFC822; luxr@263.net
Action: delayed
Status: 4.2.0
Diagnostic-Code: SMTP; 450 <lxr@foo.pku.edu.cn>: Sender address rejected: Domain not found
Last-Attempt-Date: Tue, 25 Sep 2001 20:44:00 +0800
Will-Retry-Until: Sun, 30 Sep 2001 15:56:49 +0800

------------------------------------------------------------
From: lxr <lxr@foo.pku.edu.cn>
To: "luxr@263.net" <luxr@263.net>
Subject: Fw: 003
Date: Tue25 Sep 2001 15:58:21 +0800

003