Newly created and refreshed sandboxes have the default email deliverability set to System email only.
To configure email deliverability go to Email in Administration area and select Deliverability. Set the access level in the Access to Send Email (All Email Services) using dropdown.
- No access — to prevent all outbound email to and from users
- System email only — automatically generated emails only
- All email — all types of outbound email
Setting System email only option is useful for controlling emails sent to your users or customers from sandboxes while testing new features.