if you’re using Evolution on a small pc like a notebook, you might find that the preview email pane has very little space to display the message because the message headers take up too much space.
To reduce or hide the headers shown in the Evolution Preview Pane, you can go:
Edit – Preferences – Mail Preferences – Headers
Untick the headers you don’t want to see in Displayed Message Headers
The changes apply to both the preview page and when you open a message seperately.