How can I use tabs in Mac Mail? One of the new featuers in macOS Sierra are tabs in more apps, including Mail.

How do I use tabs in Mac Mail? Clicking "View", "Show tab bar" displays a bar with one tab, but I can't find a way to open a new tab.
For tabs to work in all situations, you need to enable them in System Preferences->Dock (set the appropriate option to "always"). Afterwards, you can (in Mail) select "File -- New window" or press Opt-Cmd-N to create a new tab.