How to View the Apple Mail Outbox
How to View the MAC Apple Mail Outbox
The Apple Mail Outbox will appear automatically in the top left sidebar if you disconnect your network (WiFI or Ethernet) and send a test email from any account.
If you want the Outbox to always appear along the left sidebar then:
While the network is disconnected locate the Apple Mail Outbox, <R-click> and select Add to Favorites
To remove Outbox from Favorites reverse the process.