我公司收到客人的碗郵, 但客人收不到我的電郵

我之前可以自由寄EMAIL給客人, 客人亦可以寄絡我, 來回沒有問題!但現在我只可以收到客人的電郵, 但如果傳給客人, 會有一封電郵AUTO REPLY 說寄了2個小時都寄不出去, 會在一天內嘗試;
但一日後就說寄不出去, 請問是什麼問題, 有什麼方法可以做到我又可以寄email給客人??
AUTO REPLY 的EMAIL內容:
"
Your message has been received by smtp04b.on-nets.com but has been undeliverable to the following recipients for at least 2 hours.
The mail system will continue to attempt to deliver your message to these recipients for a total of 1 days. You do not need to resend your message at this time.

Recipient(s):
[email protected]

--- Below this line is a copy of the original message.

Received: (qmail 35597 invoked from network); 29 May 2008 11:42:10 -0000
Received: from unknown (HELO XXXXXX.com) ([210.0.227.33])
(envelope-sender <[email protected]>)
by smtp04b.XXXXXX.com (qmail-ldap-1.03) with SMTP
for <[email protected]>; 29 May 2008 11:42:10 -0000
Received: from unknown (HELO LENOVO8B0ACB13) ([218.188.206.158])
by mail.on-nets.com with ESMTP; 29 May 2008 19:42:10 +0800
From: "seanyou" <[email protected]>
To: "'Uni-Pac \(Katy\)'" <[email protected]>
Subject: TEST!
Date: Thu, 29 May 2008 19:42:03 +0800
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_0001_01C8C1C4.11FC3030"
X-Mailer: Microsoft Office Outlook 12.0
Thread-Index: AcjBgQLeowlYsEKaSGKpUcoiOBjs+w==
Content-Language: zh-hk

此為 MIME 格式的多重部份郵件。
"
請各位朋友大力幫忙!!!謝!