[초록생선]님이
 남기신 글:

>>========================================
>>
>>/etc/mail/access 파일에 RELAY 를 설정하세요. 
>
>========================================
>
>access 파일 입니다.
>
># Check the /usr/share/doc/sendmail/README.cf file for a description
># of the format of this file. (search for access_db in that file)
># The /usr/share/doc/sendmail/README.cf is part of the sendmail-doc
># package.
>#
># by default we allow relaying from localhost...
>localhost.localdomain           RELAY
>localhost                       RELAY
>127.0.0.1                       RELAY
>
>이렇게 설정되어 있거든요.
>
>외부로 메일이 전송되지 않는거 같아요. 

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http://www.linuxchannel.net/docs/550-error.txt

이곳을 참고하세요.