Use tabs in Mail on Mac
You can open multiple tabs in the Mail viewer window to easily switch between different mailboxes or emails.
In the Mail app on your Mac, do any of the following:
Open a new tab in the Mail window: Press and hold the Shift key, then choose File > New Viewer Tab. Or choose View > Show Tab Bar, then click the New Tab button .
Show all tabs: To show thumbnails of all your open tabs, choose View > Show All Tabs. In the tab overview, click a tab or click to open a new tab in the Mail window. To return to the current tab, choose View > Exit Tab Overview.
Move between tabs: Click a tab, or press Control-Tab or Control-Shift-Tab to go to the next or previous tab.
Reorder tabs: Drag a tab left or right.
Close a tab: Move the pointer over the tab, then click the Close button .
Close all other tabs: Move the pointer over the tab you want to keep open, then Option-click the Close button .
You can also work in tabs when you use Mail full screen.