Hi Florian,
nettania How can I filter the Inbox using categories? I do not find a way to display all E-Mails of a folder of a certain category. Is this possible, or not?
searching for the category name in the folder will also display the emails which have this category set. It will of course also find the emails containing the category name in other properties (e.g. subject, sender or body), but then you may apply the category filter like segro suggested.
You may also add the search to the favourites if you don't want to type it every time. Just don't forget to click on the magnifying glass icon in order to update the search results in the subsequent searches.