Error from /var/log/mail/errors:
Oct 28 16:50:40 mx postfix/spawn[28644]: fatal: unknown user name: nobody

Error from /var/log/syslog:
Oct 28 16:53:44 mx postfix/master[27737]: warning: process
/usr/lib/postfix/spawn pid 29450 exit status 1
Oct 28 16:53:44 mx postfix/smtpd[28891]: warning: premature
end-of-input on private/policy while reading input attribute name