This happens because when you send a message to your local domain, Postfix is responsible for checking that the recipient exists.

For example, if you were sending email to a user called "test", you either need to have a local user called "test". You would add the user like this:

useradd -s /bin/bash -d /home/test -U test

Alternatively, you would need an alias from test to a local user in /etc/aliases

echo "test: root" >> /etc/aliases
postalias /etc/aliases