You may want to enforce TLS for connections from ServiceNow email infrastructure to your mail servers but you cannot edit the 'Enable SSL' and 'Enable TLS' checkboxes.
Applicable to all releases.
Official support is currently non-SSL/TLS connections between the instance and the ServiceNow internal email infrastructure, and opportunistic TLS (STARTTLS) is used when the ServiceNow internal mail infrastructure initiates a connection with your external email server. This means TLS will be used only if your mail server supports TLS for encryption. Since these are email infrastructure level configurations, for ServiceNow-configured email accounts in an instance, values pertaining to the connection (SSL/TLS, host, port...) are not editable.
If you require enforced TLS, you cannot do so using ServiceNow email infrastructure. You must configure your own mail servers appropriately, and create your own email account records in the ServiceNow instance.