I am having the same issue. I REALLY NEED Roundcube to supply either an external app that we can load on our client or better yet a selection off of the MORE... menu pick that allows for Bulk Download of EMails from a folder and produces a .mbox file. Then we can use Roundcube's already-supplied Import facility. Reason why we need this where we are is that we are switching accounts to support a re-branding project for our museum.