Hello Silvio,
Why are then sometimes these «PostFinance Servers» used for confirmation and sometimes the user with his IP address and sometimes both (which then lead to double confirmation emails).
I can not figure out any logic in this.
Because sometime payments can have several statuses : "created" -> "pending" -> "confirmed" / "cancelled"
The first notification is received when your customer is redirected to your website, and the payment status can be set to "pending" or "confirmed" / "cancelled".
If the status is "pending", then a new notification needs to be triggered and it will be triggered by "PostFinance servers" by using the webhook you configured.