If you've changed your Mail layout and can't get it back, here's the fixI was using Zoom the other day (as one does), helping a friend with the Mail app on her Mac running macOS Mojave. The viewer window displayed only a list of mailboxes and a very wide list of messages. There was no third column on the right side of the window to display previews of mail messages.
No problem. Just drag left from the right edge of the mail viewer window to expose the third column. Except...it didn't work.
We tried fiddling with various menus and preferences and switching back to classic layout (remember, this is Mojave we're talking about), all to no avail.
Now what?
How the Mail app changed from Mojave to Catalina
If you've played around with Apple's Mail app in macOS Catalina, you might know that a few preferences were changed. The classic layout (list of messages on top, message preview below) was available as a check-box option in Mail's Viewing preferences in macOS Mojave:
But that option was removed in Catalina:
You can come close to emulating the class...
Source:
If you've changed your Mail layout and can't get it back, here's the fix
