Hey,
You should be receiving mail for these accounts (or root) as your self. Make sure you have done this.
sudo vi /etc/postfix/aliases
You should see the following a few lines down
…
# Person who should get root’s mail. Don’t receive mail as root!
#root: you
…
Uncomment the second line and replace “you” with your short name.
Exit and Save
Then run newaliases:
sudo newaliases
Explore the aliases file to understand what you are doing here – it is pretty straight forward.
Try sending a message to root and or postmaster at this point.