You may never see this error earlier. What is the caused?
As per Spring'13, Salesforce have added a way for you to control the type of email your organization sends, with the new Access level setting in Setup > Administration Setup > Email Administration > Deliverability.
While you typically won’t change this setting day-to-day, these options can be useful when you need to temporarily suspend outbound email or when you’re working with sandboxes, for example:
- 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.
- Newly created sandboxes default to System email only.
- Sandboxes created before ’Spring 13 default to All email.
You may not be able to edit the Access level if salesforce.com has restricted your organization’s ability to change this setting.
Summary: by change access level to "All email", it should solve the issue in Sandbox.