Customise mailboxes shortcuts
The shortcuts correspond to the order in which they appear on the Favourites Bar, underneath the toolbar. If you add, remove or rearrange mailboxes there, the shortcuts will change accordingly.

If your Favourites Bar isn't currently visible, you can show it using the View &gt; Show Favourites Bar menu.

http://www.cultofmac.com/202280/go-to-favorite-mailboxes-with-keyboard-shortcuts-os-x-tips/