Setup > Email Administration > Deliverability
Access to Send Email: To control the type of email your organization sends.
No Access: Prevents all outbound email to and from user.
System email only: Email automatically generated (user and password reset emails)
All Email: Allows all types of emails.
System email: When you create new sandbox Access Level defaults to System email only to avoid email being sent to users accidentally while testing in the sandbox. Before Spring 13 Access Level defaulted to “All Emails” which had to be changed to “System email only” while working in sandbox.