Mail.de has a very helpful postmaster site where they provide useful information for senders and receivers alike. They provide ipv4 and ipv6 IP address information for their outbound mail servers, they indicate that outbound mail is authenticated with DKIM, and they also say that they use five different DNSBLs (spam blocking lists) to filter inbound mail server connection attempts (all.spamrats.com, bl.spamcop.net, ix.dnsbl.manitu.net, bl.mailspike.net, and Invaluement). They might also use some other tools to filter spam as well.
They provide a helpful postmaster contact address, useful for reaching out if you need to resolve a blocking issue. I recently ran into an issue there, and so I reached out to that address. I provided them my IP address, domain name, and a sample blocked message, saved as a zipped .eml file (as recommended by the provider, to prevent my sample message from being blocked before they could receive and review it), and they were able to resolve the blocking issue.
If you find yourself blocked, head on over to that postmaster site, reach out to the postmaster address as recommended there, and explain the situation and include appropriate information as I describe above. Assuming that your mail is indeed desired by mail.de recipients, I rather suspect that they'll be willing to address the issue.
Be sure to check those blocking lists, too, because those could be the underlying reason behind the blocking. An ISP will often defer to the blocking list with regard to spam filtering decisions -- meaning that if you're listed on a blocking list they use, they may not be willing to override that and let your mail through. The resolution to the delivery issue to mail.de subscribers is likely to require that you address the blocking list issue.