Position your cursor on the rightmost edge of the Mail viewer window. When positioning your cursor over it, the normal selection cursor will change and bear a resemblance to this
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Drag that.
Drag that cursor to the left. There will be no visual confirmation of that action until you drag it more than halfway to the opposite edge of the window, or full screen if you are not using windowed mode.
If you prefer to use Mail's "Classic layout" the vertical column splitter becomes a horizontal one: drag up instead of left and remember you have to drag it a lot.