벨지움 거래처에 이메일을 보내면 얼마전 부터, 상대방에게는 이메일이 가질 않고
다음과 같은 리턴메일이 옵니다. 
그리고, 벨지움 거래처에서는 다음과 같이 조치를 위해달라는 이메일이 옵니다.
The informatician informs me that the IP address of your mail server must have a reverse 
dns system. That is why the emails do not go through.
Can you please check with your email server?

어떡해야 하는지요?

The original message was received at Thu, 16 Feb 2012 10:55:53 +0900
from localhost [127.0.0.1]

   ----- The following addresses had permanent fatal errors -----
<&nbsp at byus.net>
    (reason: 550 5.1.1 <&nbsp at byus.net>... User
unknown)

   ----- Transcript of session follows -----
... while talking to mail.byus.net.:
>>> DATA
<<< 550 5.1.1 <&nbsp at byus.net>... User
unknown
550 5.1.1 <&nbsp at byus.net>... User unknown
<<< 503 5.0.0 Need RCPT (recipient)
... while talking to mail.maehdros.be.:
>>> DATA
<<< 450 4.7.1 Client host rejected: cannot find your reverse hostname, [61.111.255.117]
<soliman at fashionvision.be>...
Deferred: 450 4.7.1 Client host rejected: cannot find your 
reverse hostname,
[61.111.255.117]
<<< 450 4.7.1 Client host rejected: cannot find your reverse hostname, [61.111.255.117]
<jose.noe at fashionvision.be>...
Deferred: 450 4.7.1 Client host rejected: cannot find your 
reverse hostname,
[61.111.255.117]
<<< 554 5.5.1 Error: no valid recipients