Why did I receive an email about setting up a digital mailbox when I already have one?

When public agencies send out emails encouraging the public to set up a digital mailbox they use the contact information registered in the common contact register.

If you have registered your email address on behalf of another person, the email will be sent to you. Perhaps you have helped a family member or are a guardian. You may have provided your email in connection with this. For example when logging in to the tax return.