all modern email clients offer a unified inbox. So far so nice. Others offer the option, that the user can decide which accounts should be displayed in the unified inbox, which not. Is there an option for this in EM Client as well. At least I could not find it.
If you want to create your own custom selection, you can right-click on Search Folders and choose New Search Folder. Then give it a name and choose what folders you want it to display.
If Search Folders are not visible below the Favorites, you can enable them by choosing Menu > Settings > General > General > Show Search Folders.