Taking Mail full screen can be a good way to focus and crank through your inbox. Fortunately, you can set it to open messages and new drafts in an adjacent Split View, which keeps your message list, a separate viewer, and your mailboxes still accessible. The message side of Split View will even open multiple messages and drafts in tabs.
Check out the gallery below to get the idea.
To enable this Split View behavior, to go Mail’s Preferences > General pane and toggle Prefer opening messages in Split View when in full screen. I know, the name doesn’t exactly roll off the keyboard. But it’s a handy option that I’d like to see in more Mac apps.