In my case it appeared in the logs for two files:

postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postqueue
postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postdrop

fix is quite trivial though:

•  chmod g+s /usr/sbin/postqueue
•  chmod g+s /usr/sbin/postdrop

Hope this works for you as well.