We have aggregated mails into one within a 10 minute period for the following notifications:
- new task assigned
- you are no longer assigned to tasks / task unassigned
- task due date changed
- task is due soon
- task overdue
- task reassigned to the group
- task rejected
The ability to individual users to decide which notifications to get or not is on the backlog. In the meantime you could use E-Mail application filters to filter out any e-mails you don't want to see.