This is extremely interesting, can I sync my Tasks.org ToDos with Posteo?
Edit: From what I understand, Tasks.org uses CalDav for ToDos which Posteo supports, but (and it sounds silly) do I have checkboxes in my calendar or something like that to know if a certain task is done or some way to see the ToDos in the same way I see them in Tasks.org? I hope they don’t look like just an event in the calendar.