The single most frequent question we get is "Will John Doe be notified if I enter this?".... and the answer is always "I don't know, depends on how he has his emails configured".
At this point, most people email out side the system in parallel to be certain important communication gets noticed.
Suggestion - Add a little info button that will run a check of "Who was emailed" next to all comment updates and system updates.
This would act as a fail safe check to confirm a complete list of users that received an email notice (or daily digest), after accounting for their notification settings.