Enable "Download Messages for Offline Viewing and Searching" to access your mail without internet connection.
Right-click on a contact in the Contacts Panel and select View All Messages to see all emails they've sent.
Press K, G or M on your keyboard to mark messages in Vivaldi Mail and Feeds as read.
Create a Mailing List from an email address for quick access to all emails from the sender.
Enable threaded view of messages in Vivaldi Mail with a click of a button.
Reorder sections in the Mail Panel to your liking.
Enable Mail, Calendar and Feeds from Settings > General > Productivity Features.