Jon von Tetzchner on vivaldi.com/blog/:
We are excited for you to test the new Vivaldi Mail. Some people fondly call it M3 and there is a bit of history to this that I’ll share later.
In fact, I’d admit that one of the reasons for Vivaldi’s existence is to provide a browser with a built-in email client. And today we have the first glimpse of it.
The cornerstone of Vivaldi is the database. All your mails from all your accounts are indexed, therefore searchable offline. If you prefetch your mails, we index all the content of the mails as well even before they are opened.
Used to love Opera’s M2 — even became an annoying purist that wouldn’t open internal mails that weren’t plain text while I worked there.
Then the convenience of Gmail won me over. But many mail clients later (currently Superhuman), I’m always game to try a new old one.