For the benefit of others we have two different editors for Email. The newer, ONTRAmail editor uses a single scroll for the page and the arrow keys work as expected. The older HTML Email editor does have a scroll bar for the editor window and the page itself.
You can make the HTML Email editor full screen to solve this problem for lengthy emails. It is this button on the tool bar:
It works in both Source and Regular view, and you can toggle the display back and forth. Here's the full toolbar at the top of the full screen after clicking the button. Click again to go back to the regular page view :
Other tips on using the HTML Email editor are found in our HTML Email article.