That error means postfix has no read/write permissions for queue directory /opt/zimbra/data/postfix/spool
See file /opt/zimbra/postfix/conf/main.cf and locate "queue_directory =" to see your current queue directory.
Most of the user run chown -R zimbra:zimbra /opt/zimbra
Which creates this problem.
To fix run and restarted zimbra
chown -R postfix

ostfix /opt/zimbra/data/postfix/spool