We had a very similar issue.
We have set users email notifications to either 'none' or 'daily digest' in each persons settings (People module), the notifications field can be shown in the list view to aid this change.
Once we had done this we then implemented the Resource Notifications app from the marketplace. This means that users receive and email when they are allocated a task (Including email users) and you can then control better the amount of emails being generated. You might need to adjust the settings in the workflows for generating the emails to suit the exact information you wish to send.
The app is http://www.clarizen.com/appsmarketplace/item/Resource-Notifications.html
Hope this helps