SMTP defer vs reject

Solution 1:

It is failsafe feature from postfix. Imagine that you just have postfix installed and then point your MX record of example.com to postfix before you include example.com to your postfix configuration. If the default parameter is reject_unauth_destination, all email going to example.com will be permanently rejected.

In short, change the default configuration defer_unauth_destination to reject_unauth_destination when you are sure that all domains belongs to your postfix has been included to the configuration.

Solution 2:

Postfix and generally all mailservers defer temporary failures. Temporary failures have 4.x.x code. There are permanent failures that have 5.x.x code. Mailservers don't retry permanent failures by default.

Access readme suggests that for a reason. If unauth_destination failure happens it probably will happen if you retry, It's more like a permanent failure that won't go away on it's own over time.