The domain yahoo.de is controlled by whoever currently controls the Yahoo! brand. The mail server for this domain is also controlled by the same party. Any mails which should be delivered to this domain will check for the MX of the domain and use the given mail server to deliver the mail, no way around:
$ host -t mx yahoo.de
yahoo.de mail is handled by 10 mx-eu.mail.am0.yahoodns.net.
This owner of the domain explicitly states the policy regarding the domain name in the email addresses:
Most Yahoo accounts end in "@yahoo.com". The domain extension is the ".com" part. Some older accounts have a different extension depending on the location and language where they were created. You can no longer create accounts with Yahoo domains other than "@yahoo.com".
If you have a Yahoo ID with a different local domain, like "@yahoo.co.uk" or "@yahoo.fr", make sure you include the correct domain extension–the ".co.uk" or ".fr" part–when giving out your email address. If an email is sent to the wrong domain, it won't be delivered.
Thus, if this email was created in the past it might still work. But you will not be able to create a new email with this domain and you cannot use any tricks to somehow use such an email but deliver it to a different account, at least as long you don't control the system sending the mail (which you likely don't).