How did you find out it is from kundenserver.de?
In gmail when you view a message, you have a small downwards arrow at the top left of the message next to where it says 'to me'. If you click that it will say 'mailed by:' and give the server name.
You can also click 'Show original'. In other email clients the option may be 'show headers'.
Another tell tail sign is if when you hover over a link or a button, the status bar says the link is on googledrive or a domain which is not google.
These messages are crafted to shock and alarm you so that you click the button without thinking, always check, and if you are unsure ask somebody technical to take a look before you do anything.