A special option called "Valid Sender Domain Only" is present on top of your domains screen.
When this setting is enabled on your account or subaccount, a verified domain has to be used for sending. If it is detected that an email is being sent from a domain that is not properly verified on your account, such email will be suppressed with following reason:
"Valid Sender Domains Only is enabled and the sender domain is not validated. Please see your domain settings to control this behavior."
When you are a parent (main) account, you can also set up this limitation for any of your subaccounts on the Edit Sub Account screen:
This setting will allow your subaccount to send only with use of their own verified domain. They will not be able to use your main account's default domain as sender. When attempted, they will be suppressed with the same error as in the case of your main account.
You can read more about domain verification here