From documentation

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 can’t edit the Access level if Salesforce has restricted your organization’s ability to change this setting.

to get more details about it, you need to create a salesforce support case

From documentation

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 can’t edit the Access level if Salesforce has restricted your organization’s ability to change this setting.

to change it to any another desired value, you need to create a salesforce support case