[Dovecot] Can't change permissions for /var/spool/postfix/dovecot.index.log

For some reason i can't change the permissions on /var/spool/postfix/ dovecot.index.log from postfix to dovecot, or anything else.

I changed all the files in /var/spool/postfix to be owner postfix so
that postfix wouldn't complain about insufficient priviledges.

I'm running MacOS X 10.4.7.

I can change it all I want, but the owner and group changes do not
'stick' - they simply stay at postfix.

Does anyone have any ideas?

Can I have the index and index.log files elsewhere?

Thanks,

James.