I get the same issue too, well, when it comes to the error. This happens to me when there's around over 20 emails in the queue, and I get a lot of the same error (maybe around 30 above but the queue looks fine after scrolling through all of that).
The list build up originally was to do with the server attempting to send emails to the root email address that doesn't exist. I now have disabled email notifications from CSF.
In not-so relation to the actual error, I had to modify /etc/postfix/main.cf so that the 'mydestination' directive would only have 'localhost' as its value (so 'mydestination = localhost'). That fixed the sending where the email would end up in spam and sorted out receiving.