DAVx⁵ has been successfully tested with SmarterMail.

SmarterMail Business Email and Collaboration Server is a scalable business email, live chat and collaboration server that meets the needs of any sized business. It runs on MS Windows and is an MS Exchange alternative.

DAVx⁵ settings

Base URL: /WebDAV/ (e.g. https://server.example/WebDAV/)

Contact group method: groups are per-contact categories