Provide means to disable e-mail notifications for a user.

Given the following situation:

• Employees have left the company, and their email address is no longer valid
• Their accounts in crowd have been marked as inactive, and they have been removed from all groups, thus they are no longer active users.
• We can't delete the users since they are the Reporter/Assignee/etc. of many current and legacy issues.
• When an issue they are the Reporter/Assignee on is updated in a way that a notification is generated, the email is bounced (since it is no longer valid) back to the project's email, a mailing list for the dev team.

Typically in our notification schemes, the Reporter and Current Assignee are notified upon updates. I do not want to remove these users from those designations.

It seems like it should be a trivial enhancement to simply either:

• provide an option to not send email notifications to users that are not active, or
• allow users (administrators only?) to enter a blank email and have the system not notify users without email addresses.