This has been occurring since the last app update, and Samsung has not even acknowledged the issue. There have been multiple posts regarding this issue to date with several suggested fixes, but to no avail. Although some suggestions appear to show the issue as being resolved check your battery usage for the app, and it will show background for the time your phone has been on, as will total usage time.
It's because there's not an actual issue. The email has to run constantly in the background to stay synced to the server so that it can constantly pull in updated incoming emails. - This way you get all of your emails in real time.
If you don't like it doing it just go to Settings>Apps and force stop the email application.
