Hi Jonathan,
At this time, there's not a setting available to switch this behavior. This functionality was changed with the release of SmarterMail 16.x. That said, I'll go ahead and change this thread from a Question to a Proposed Idea, and I'll forward this request to the development team for their consideration. I agree with the convenience of previewing the selected message when clicked.
Thanks,
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com