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:
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.