샌드메일 어느날부터 갑자기 안되는 현상

안녕하세요..

몇일 전부터 갑자기 샌드메일이 보내기가 안됩니다.

외부-외부 만 보내지구요..외부-내부,내부-외부 안됩니다.

/var/spool/mail 에서 확인한 결과 아래와 가팅 NDR이 나타납니다.

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The original message was received at Thu, 12 Jul 2007 09:01:12 +0900
from localhost [127.0.0.1]

   ----- Transcript of session follows -----
... while talking to rcvmail3.naver.com.:
<<< 421 4.3.2 Your ip is filtered by RBL (http://realip.naver.com/rbl/rblinfo.html)

(Caues:xxx.xxx.xxx. See http://www.dnsbl.sorbs.net/cgi-bin/lookup?NAME=
... while talking to rcvmail1.naver.com.:
<<< 421 4.3.2 Your ip is filtered by RBL (http://realip.naver.com/rbl/rblinfo.html)

(Caues:xxx.xxx.xxx See http://www.dnsbl.sorbs.net/cgi-bin/lookup?NAME=
... while talking to rcvmail4.naver.com.:
<<< 421 4.3.2 Your ip is filtered by RBL (http://realip.naver.com/rbl/rblinfo.html)(Caues:xxx.xxx.xxx See http://www.dnsbl.sorbs.net/cgi-bin/lookup?NAME

RBL에 막혀 있는것도 아닌데...이런한 오류가 어떤오류인지 아시는 고수님들 알려주세요..ㅠㅠ