Email Deliverability

New and refreshed sandboxes use the System email only setting by default. Sandboxes created before Spring ’13 default to All email.

Required Editions

Available in: both Salesforce Classic and Lightning Experience

Available in: Professional, Enterprise, Performance, Unlimited, and Database.com Editions

User Permissions Needed
To view a sandbox:	View Setup and Configuration
To create, refresh, activate, and delete a sandbox:	Manage Dev Sandboxes (Developer or Developer Pro only) or Manage Sandboxes (all sandbox types)

This setting applies to production and sandbox. To set it, from Setup, enter Deliverability in the Quick Find box, and then select Deliverability. If editable, set the access level in the Access to Send Email section. If Salesforce restricted your ability to change this setting, you can’t edit the access level.

No access—Allows only password reset emails. Prevents all other 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 orgs that aren’t sandboxes.

Tip The System email only setting is useful for controlling email sent from sandboxes so that testing and development work doesn’t send test emails to your users.