Universal Containers is piloting new features in an existing sandbox and wants to prevent outbound...

 Setting email deliverability to no access would prevent outbound email sends during testing in a sandbox. This would ensure that no emails are sent to any users or external email addresses from the sandbox. Setting email deliverability to system email only would still allow some emails to be sent, such as password reset emails or new user emails. Configuring SMTP authentication or email relay would not affect email deliverability in a sandbox