One nice thing in SM15, Thunderbird, Outlook, etc. is the ability to resize the window panes (folder, inbox, & preview) so that you can maximize what you see in your browser. That doesn't work in SM16.
If it can't be fixed, I propose reducing the width % of the folder pane and increase the width % of the preview pane. Many messages are not responsive and it's a pain to scroll right/left to read them in the preview window. It's especially bothersome when there's a lot of empty space in the folder pane.