When sending emails from NetCommunity, a transparent image is added to the end of the email that allows the software to determine if the email has been opened or not.  If an external DNS record has not been created that matches the Site URL configured in NetCommunity, the user's email editor will not be able to download this image, which in turn will not allow NetCommunity to track whether the message has been opened or not.

Create a DNS record for the Site URL that is accessible to the internet, and all future emails that are opened should now track correctly.