Postfix - centOS 7

I'm currently trying to install and make Postfix, dovecot works.I've followed this tutorial right hereBut right now, I'm facing a "permission denied" error:tail /var/log/maillog shows me:

warning: mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf is unavailable. open /etc/postfix/mysql-virtual-mailbox-maps.cf: Permission denied

The permissions for all these mysql-*.cf files are:

ls -ld /etc/postfix/mysql*.cf:-rw-r-----. 1 root postfix 151 Apr 17 18:07 /etc/postfix/mysql-virtual-alias-maps.cf-rw-r-----. 1 root postfix 142 Apr 17 18:08 /etc/postfix/mysql-virtual-email2email.cf-rw-r-----. 1 root postfix 139 Apr 17 18:05 /etc/postfix/mysql-virtual-mailbox-domains.cf-rw-r-----. 1 root postfix 138 Apr 17 18:06 /etc/postfix/mysql-virtual-mailbox-maps.cf

What permission should these files have? I don't understand why it denies it.

Thank you in advance!