Hi @kparzinger
Technically, your configuration is correct. That said, some mail servers expect a Fully Qualified Domain Name (FQDN), so whatever.randlweb.de could be a better solution. Just change your server’s name to whatever.randlweb.de, point whatever.randlweb.de to your public IP, and update the PTR record of the public IP to point to whatever.randlweb.de.
That’s all, you don’t need to modify any other domain or worry too much about the PTR record.