How do I unhide the Mail preview pane? In my default view when using Apple's Mail on macOS, I used to see 3 coluns, from left:


Mailbox
Inbox
Email


I stretched the Inbox column / pane to width of the full window, and now have only 2 columns:


Mailbox
Inbox


I cannot "pull back" the Email pane with a mouse.

How can I restore the missing Email preview pane?
You have moved the divider line to the right to set the email pane to zero width. You move it back the same way, but you have to move to almost full width for it to "hold", after that you can vary the width as you like.
It is the same in Classic View: if you do not move it up very high it will not hold.