Because eM Client is a Windows program, it supports CardDAV. Via “Extras” > “Accounts”, a new “CardDAV” account can be set up for “Contacts”.

For the URL, please enter the following, changing “john.example” to your Posteo username:
https://posteo.de:8843/addressbooks/john.example/default

The username is your Posteo address. For the password, please enter your Posteo password.

Choose a description and then click “Complete”.

In the address book, synchronisation occurs by clicking the “Refresh” button.