MAIL Question ? Switching From A Full Screen View; How Please ?

Hello,

Using the MAIL App that came with my iMac. (Mojave)

Like it, and am quite satisfied with it, but it does have

its "quirks".

Now and then, the MAIL screen takes up the entire Mac screen.

How do I get it back to something reasonable, and

re-sizable ?

Is there a Key that switches to full-mode, and back again ?

Thanks,

Bob