mail-mta/postfix-2.9.0 is broken. When started or stopped is tries to access /usr/lib64/postfix which doesn't exist. Probably this should be /usr/libexec/postfix instead.
Had same problem. Fix is to change: "daemon_directory" in "/etc/postfix/main.cf" to "/usr/libexec/postfix".
Fixed in postfix-2.9.0-r1.