We got many request in success community saying not able to change the “Deliverability” option in sandbox org.
By default you will have “System email only” after the sandbox refresh. If you are not able to change please contact salesforce support to enable the option.
Salesforce Document says :
New and refreshed sandboxes have the default email deliverability setting System email only. To configure email deliverability settings, in the sandbox org, from Setup, enter Deliverability in the Quick Find box, then select 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 Salesforce has restricted your organization’s ability to change this setting.
After enabled the feature