@bazinga the app can currently import and export in vcard/ical formats, so that should work fine. There is no automatic migration implemented though. For Roundcube, is far as I understand, it has no easy way to use external addressbooks. Our contacts app does export addressbooks via carddav, so thunderbird and such do work though.
@aartjansen we are not aiming for mainly to build features SOGo does not have, but mostly because the apps like roundcube and SOGo are next to impossible to fix or contribute to, if need be, so this is an ongoing risk from Cloudron perspective, as those basic use-cases are crucial for most users.
For the out-of-office, could you please elaborate a bit more on what this would involve from your perspective?