>>Why don't you disable Update for "Sensitive messages"?
>
>I didn't think about that so far. I would have to see with our team to see if that would be better or worst if we could implement such flexiblity.

As I see it, you've only two choices:
- Disable update
- After each update revise it to see if message is still 'Sensitive' and remove designation if it's not. It doesn't make any sense to keep message marked as 'Sensitive' if it's not anymore.