Clear The Ring!
You go to a circus and when each act is over, something used in the act is left behind. Mid-way through the circus performance they would have to stop the action to clean out the stuff lying around in the wing.
We do something very similar when we leave read emails in our inbox. Each time we go to manage our inbox, a portion of our focus or attention is always drawn back to those emails sitting idly by. We mentally trip over them while trying to really focus on our new acts.
Why not create some rules for incoming emails or better yet, create some rules for what you do with an email once you read it? An old acronym to guide management of pieces of paper still works for handling email. It is:
Remember, the goal is to have your mental "ring" clear to be able to focus on the act at hand-and not trip over it.