Vivaldi Mail has three read states – Unseen, Unread, and Read. Messages go from Unseen to Unread automatically after opening them, but marking each message as Read is an active choice. The Unread state is a good way to keep track of messages you’ve already seen but have not dealt with yet.

If this doesn’t fit your workflow, you can skip the Unread state by enabling “Automatically Mark as Read” in Settings > Mail > Mail Settings > Unread Messages.