Looks exactly like the old opera email client, which i guess is what they were going for. Opera was excellent in the presto days so perfectly good thing to emulate.
Nice thing, normally implemented: you also can customize toolbar to have mail notificator icon right in your top toolbar without statusbar, for example. Memory usage is not greatly increased during email indexing. But: no support for protocols < TLS1.2, so I just can't use it for work mail for now.. Anyway - interesting product, like in old good Opera!
Add a phone dialer and SMS client and Vivaldi could almost run a linux phone.