In that case follow x64uk's instructions.
Another description is repeated in the Discussion I linked:
John Galt wrote:
Refer to the rightmost part of the window, where you last dragged the left pane all the way to the right. When positioning your cursor over it, the normal selection cursor will change and bear a resemblance to this
<-|
Drag that.
Drag that cursor to the left. Drag it farther than you have been doing, much farther. 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 are using 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.