When I have restarting the postfix server I am facing the below error ::
postfix-out/master: fatal: open lock file pid/master.pid: cannot create file exclusively: Permission denied
I having a centos server.
This file is created every time postfix starts.
In debian the file is created in /var/lib/postfix/master.lock, you have to looking for the directoyr of your postfix an check the permissions and the owners of it.