How do I use tabs in Mac Mail on El Capitan? I want to use tabs when composing new emails in Apple Mail on my Mac. I searched here and read this question:

How can I use tabs in Mac Mail?

The thing is this question says it’s a new feature in macOS Sierra, but I am convinced that in the past I used tabs in Apple Mail. I know it wasn’t in Sierra because my Mac runs El Capitan and has never had Sierra installed.

Did I just imagine this? Was it a dream? Or is this actually possible in El Capitan too?
Yes, it’s possible. You just need to run Mail in full screen mode first. You do this by clicking on the green button at top left of Mail’s window. 

Once in full screen mode, create a new mail message as usual. This opens the New Message window, but now whenever you do commandN it’ll add a tab to the existing New Message window for your other new message.