Preventing sandbox from sending Workflow emails to "Additional Emails"

From the Help Docs

Newly created sandboxes have the default email deliverability setting System email only.To configure email deliverability settings, in the sandbox organization, click Your Name | Setup | Email Administration | Deliverability. If editable, set the Access level in the Access to Send Email section. You may not be able to edit the Access level if has restricted your organization’s ability to change this setting. No access>: Prevents all outbound email to and from users. System email only: Allows only automatically generated emails, such as new user and password reset emails. All email>: Allows all types of outbound email. Default for new, non-sandbox organizations.

Sandbox Setup Tips and Considerations