[求助] 請高手幫忙!

請問網主, 以下的錯誤是 sender 方面的 server 或是 receipant 的 server error?
是那裡出錯?

The original message was received at Tue, 28 Feb 2006 13:25:23 +0800
from nobody@localhost

----- The following addresses had permanent fatal errors -----
xxxx@xxxxgroup.com
(reason: 554 5.7.1 This message has been blocked because the return email domain is invalid.)

----- Transcript of session follows -----
... while talking to mail.xxxxgroup.com.:
>>> DATA
<<< 554 5.7.1 This message has been blocked because the return email domain is invalid.
554 5.0.0 Service unavailable