[apristel]

You can't enable/disable the SMS Sent notification in the notification settings. You need to do it with a registry edit or a program that does it for you ( advanced config )

If you're tired of the "Message Sent" notification after sending an SMS/text message, you can disable them:
HKLM\Software\Microsoft\Inbox\Settings\SMSNoSentMs g = 1 (DWORD decimal)
And to re-enable:
HKLM\Software\Microsoft\Inbox\Settings\SMSNoSentMs g = 0 (DWORD decimal)

Please note that the 'Settings' key may not exist in your device, and thus may have to be created first.