CNAME configuration is mandatory to send emails from your account. It is advised to create one before you start sending emails via Netcore so as to enable Netcore application to replace the envelope domain and URL subdomain within your emails with your own subdomain configured within CNAME record on your Netcore account for tracking of user engagement in terms of opens & clicks.
These settings are used for tracking of bounces and also to track the user actions against the emails sent like Opened, clicked, unsubscribe..etc This is as per the globally followed standards of good email senders and it helps build a good reputation of emails along with good inbox delivery within top ESP’s like Gmail, Yahoo..etc.