Have you tried the registry edit from WM5?
http://forum.ppcgeeks.com/showthread...able+text+sent
Disable the SMS Sent notification
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.