
FastMail is an email service offering paid email accounts. It also allows to manage calendars and contacts and provides CalDAV/CardDAV interfaces.
App password
In order to use 3rd-party applications like DAVx⁵ or Thunderbird with FastMail, you need to generate an app password. Choose "Mail, calendars and contacts" access.
DAVx⁵ settings
Choose "Login with email address" and enter your FastMail email address and the generated app password (see above).
Contact group method: groups are separate VCards