I generally subscribe to the GTD/inbox zero approach mentioned in one of the other comments. In this system, after reading an email it should be processed into

I generally subscribe to the GTD/inbox zero approach mentioned in one of the other comments. In this system, after reading an email it should be processed into your task management system, unread emails are waiting to be processed and read emails have already been processed, so no need to worry in either case. To process an email means to read it and determine what action is necessary and, if relevant, file in the task system. The possible outcomes from reading an email are: Do, Defer, Delegate, or Archive. You should Do something immediately if it’s very urgent or quick to do. Otherwise Defer the action for later by saving a task in your management system (not in you inbox). Delegate and Archive are pretty straightforward. Read up on GTD method for more details.