Vivaldi is a web browser that can be downloaded for free on Windows, Mac, or Linux. Vivaldi Mail is Vivaldi's built-in mail client, allowing you to keep your mail folders in sidebar view while browsing the web.
Vivaldi Mail needs its own app password to access your information. If you use your normal password or your Fastmail two step verification password in Vivaldi Mail, syncing won't work and you will see a password error.
How to set up Vivaldi Mail
1. Download and open the latest version of Vivaldi.
2. Click on the Vivaldi Mail icon on the browser's sidebar.
3. Click Add Mail Account.
How to set up Vivaldi Calendar
To add your Fastmail calendar in Vivaldi, open the Calendar tab in Vivaldi's sidebar. You'll be prompted to click Add Calendar Account to add your first calendar.
Select CalDAV from the sync options listed and then click Continue.
Enter your full Fastmail username, app password, your desired name for the calendar account, and the server https://caldav.fastmail.com/
and then click Add Account.
For additional information on adding and managing calendar accounts in Vivaldi Calendar, check out this Vivaldi help page.