I'm glad you asked, since this is something I'm currently working on (I'm a developer at Runbox). So currently our backend for contacts storage is home-baked,

I'm glad you asked, since this is something I'm currently working on (I'm a developer at Runbox). So currently our backend for contacts storage is home-baked, and pretty much exclusively accessible to our webmail. This works fine for web usage (though the Contacts view in the new, opensource client is a bit basic right now), but is exclusively accessible to webmail and not really synchronizable with other devices or DAV servers. So the goal is to migrate the existing contacts database to DAV and change the webmail to use the DAV server instead of our home-baked database. We'll also provide direct access to the DAV server to our customers, so there can be a two-way sync between Runbox webmail and whatever other client/device you desire. It will also be possible to use whatever DAV server you want with the webmail (for example if you have your own existing nextcloud that you want to reuse). Once the entire DAV infrastructure is in place and battle-tested, calendar should follow contacts Soon[tm].