Haven't checked it out yet, but IMO if the mail notification system is enabled, it should work as follows:
In an Issue, an email should be sent upon creation of the Issue or on any change, to everyone who is:
- System Admin (configurable)
- Project Admin (configurable)
- Exists in the "Assigned to" field of the issue
- Has made at least one modification to the issue
- If someone has just viewed the issue, he should not receive mail notifications unless he has manually subscribed to it.
It would also be convenient to be able to adjust the system to send i.e. one email per X minutes to each user, containing links to all issues that meet the above conditions.