In order to lessen the amount of email notifications it would be good to have a digest style notification system that can run off multiple queries created by the user.

The plugin here:
http://www.redmine.org/boards/3/topics/27087

Seems to fit the bill, but is behind the current version and there is not much work being done to keep it up to speed. The plan already set out by this plugin would provide the basic functionality.

• Provide a cron like interface for timing notifications - alternatively provide a service that can be run manually which will action the waiting notifications
• User configurable queries to create notifications based on user requirements
• (option) customization format
• Can be used on a 'per-project' basis to provide flexibility for project managers