You need to appreciate what mout.kundenserver.de is..
This is a mail server delivering mail for customers who use 1&1 to host their email and/or as a domain registrar which forwards their email. So if one domain is seeing delays, every other domain managed by 1&1 will also experience the same delays.
Furthermore mail delays during SMTP relay are usually a result of problems caused by the receiving, rather than the sending relay. Let me give you a real world example.
I have a server that uses Greylisting. Thus when I receive a mail from a new sender my server sends out a temporary error 4xx code.
When the sending party sees this code from my server it queues the mail and tries again at various intervals until either
The message is delivered
The message queue time is exceeded - (in which case the mail is bounced back to the sender)
Note: It's only when the message is delivered that my server will add it's Received: and other headers, which is why it is easy to mistake the sender for the delay..
I do recommend that you ask Liberty Global to look into why they are delaying mail from 1&1 Ionos.
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