Their service may be good enough but they are hardly investing enough into their product at the moment. mailbox.org is also not a core product of theirs. Just read through the forums (mostly in German); there are basic features/aspects missing you'd expect with a paid service nowadays like automated DKIM next to bug fixes needed around the web interface, which also can't compare to Fastmail's or Google's mail UI in terms of efficiency and usefulness – it's clunky: consequences of being fully dependent on foreign upstreams, I guess. Plus there's plenty of downtime. And that's for prices similar to Fastmail and GSuite when looking at similarly featured tiers (GSuite Basic – Fastmail professional – mailbox.org Mail XXL plan).
I actually ran mailbox and fastmail side by side before settling on mailbox. I think the UI is next to flawless. AFAIK they don't work on the UI themselves, they use https://www.open-xchange.com/
Well that's fair enough, I basically never use the web interface so I probably haven't ran into those issues. For me it's about where they're located and their customer service more than their interface.
