Openwebmail都會出現下面的錯誤訊息

請問前輩~~
小弟架設了一個openwebmail而它能寄信給對方
但對方卻無法寄給我的Openwebmail,請問這可能會是什麼造成的~~?

Thank's~~!

以下是錯誤訊息~~~~~~!!!!!

----- The following addresses had permanent fatal errors -----
<[email protected]>
(reason: 550 Host unknown)

----- Transcript of session follows -----
550 5.1.2 <[email protected]>... Host unknown (Name server: 192.168.0.20: host not found)

Final-Recipient: RFC822; [email protected]
Action: failed
Status: 5.1.2
Remote-MTA: DNS; 192.168.0.20
Diagnostic-Code: SMTP; 550 Host unknown
Last-Attempt-Date: Tue, 18 Jun 2002 14:52:16 +0800

寄件者: "aaa" <[email protected]>
收件者: [email protected]
主旨: test~~************
日期: Tue, 18 Jun 2002 14:52:15 +0800