If you see a message in your account that says "It appears the email you provided is owned by another person," it means that an email from Scratch has been marked as spam, either manually or by a firewall.
If you have a Teacher Account, any associated student accounts automatically have your email attached to them. That means when your email was blacklisted, every account associated with that email was sent a notification.
Please reach out to the Scratch Team using Contact us to remove your email address from the Do Not Send list, but if the issue is with a school/district firewall, you may need to take further action. If you have access to your school's IT department, I would recommend asking them if they can whitelist no-reply@scratch.mit.edu. This will ensure that you can get automated emails from Scratch in the future, which you'll need for things like changing passwords.