Deactivate auto-response email to customer on case creation

I want to deactivate the auto-response email that seems to be sent automatically by Salesforce on every case creation. I have deactivated all auto-response rules I could find, but still get those messages, something similar to this (translated from German):

Subject: Your request is being processed

Message Text: Hello, a member of our customer service team has been assigned to your customer case number 000xxxxx, You will hear from him shortly. Thank you very much!

000xxxxx stands for the newly created case number.

So where can I deactivate those notification emails to our customers?

Thanks in advance

Frank Merz