fatal: Queue report unavailable - mail system is down

mail postfix/postqueue[20397]: fatal: Queue report unavailable - mail system is down

If you are getting this error while sending mail via Postfix, Restart the postfix.

$ sudo service postfix restart

Now try to send a mail.

If sendmail and postfix both are installed on the system, stop sendmail and try again with postfix.

If you are still getting same error. Remove and install postfix again.