Mailfence is a secure and private mail service, that provides an end-to-end encrypted email with digital signing.
Contact group method: groups are per-contact categories (contact groups are mapped to Mailfence contact tags)
User name and password: your Mailfence login data
You can mount folders from Mailfence Documents (virtual drive) using the DAVx⁵ WebDAV mounts feature.
https://mailfence.com/docs/<user name, all lower-case>
User name and password: same as for CalDAV/CardDAV
Some operations like video streaming won't work because the server doesn't support range requests.