As the mail app developers don’t want to add a horizontal reading pane to the app I have been using the custom css app in order to do so, I just wanted to share what I have managed so far for anyone else who is interested and get help finding/fixing any issues.
Currently I have:
The basic Horizontal layout
Added a resize to the message list so you can drag it up and down for simpler management of mail
Added a yellow highlight to The images have been blocked to protect your privacy notification
Added the color #ebebeb as the message header background to better separate panes
Added a thick border color #ebebeb to the top of the reply field to better separate it from the current message