1. Due to the high contrast of the navigation column on the left, I didn't realise that it belongs to the program at all. Therefore, I didn't find the 'writing new mail' button.
2. there are too many columns (navigation, folders, inbox, opened mail) that are too similar and not well to discriminate.

Solution:
Change of contrast/colour
-> Change background colour of the Folders Column to a light gray -> plus: navigation column becomes more discriminable