Wierd sendmail access file problem

Hi, If I put this line in my /etc/mail/access file sinamail.com 550 Unknown user I get makemap: /etc/mail/access: line 153: key sinamail.com: put error: Operation not permitted Now I can put anything else in the file and things work like they should but this causes things to choke. Is sinamail.com some sort of code word??? Thanks Nick