As far as I know, LibreOffice Base doesn't have this exact functionality. While Base can do master-detail relationships through subforms, it can't replicate that neat '+' expansion UI that Access provides where you can drill down into related records inline.

I've used both systems extensively, and this is honestly one of Access's killer features, especially when dealing with hierarchical data like your mailing list example. The Base alternative would be to create separate linked forms or grids, but it's definitely clunkier compared to Access's elegant expand/collapse approach.

If this hierarchical view is a core requirement for your workflow, you might need to stick with Access. While LibreOffice Base is great for many database tasks, this specific UI pattern isn't something it can match yet.