The "Mail owner" option specifies the owner of the Postfix queue and of most "Postfix daemon processes". Specify the name of a user account THAT DOES NOT SHARE A GROUP WITH OTHER ACCOUNTS AND THAT OWNS NO OTHER FILES OR PROCESSES ON THE SYSTEM. In particular, don't specify nobody or daemon. PLEASE USE A DEDICATED USER. This option sets the "mail_owner" postfix variable.
The "Default privileges" option specifies the default rights used by the local delivery agent for delivery to external file or command. These rights are used in the absence of a recipient user context. DO NOT SPECIFY A PRIVILEGED USER OR THE POSTFIX OWNER. This option sets the "default_privs" postfix variable.