Improving web interface response time
I live in Australia so this issue may only pertain to people outside of Germany/Europe. But the response time of the mailbox.org office portal is extremely slow (to the point that opening a 10kb text document can take ~15 seconds - much longer than simply downloading a 10kb file from somewhere in Germany!).
The fact that this takes so long suggests that the software for reading the text document is taking time to either send to my browser or to load on the server side. If the former is the case I suggest that as soon as somebody logs into the portal, all the necessary office software is loaded into the users browser in the background, so when they go to click on a function it happens much faster.
To illustrate further, currently logging on to the web interface will take ~10 seconds for example, but even after that, when I click the button to go to my inbox it takes a further few seconds, then if I click on the calendar it's another few seconds. After something has been selected once, however, it is fast to load upon clicking. Thus I think if things can be preloaded in the background users won't have to suffer waiting through something that should be relatively instant in 2020. And note other European email services have much faster response times for their web interfaces too.
Other than that, great service guys. I wish you great success!