Low delivery rate due t-online.de domain issue

Hi, Need your support here.

I am reaching out to you regarding an issue with the t-online.de domain that we are experiencing. We have a relatively low delivery rate of 86,5% for one the recent campaigns (96 out of 710 have been blocked). Almost half of the undelivered emails have been rejected with the error message: 554 IP=a.b.c.d - A problem occurred. (Ask your postmaster for help or to contact ...@... to clarify). The common denominator of these emails was the t-online.de domain, so we contacted the email provider to better understand the issue. Please find below their response:

"(...)Please note that hosts which are de facto anonymous to third parties are not qualified for delisting at our system. See the information in our "Postmaster FAQ" at https://postmaster.t-online.de/index.en.html, especially chapter 4.1: A mail server's IP address must have an FQDN (PTR-Record, rDNS) in a domain that belongs to the operator of this mail server. This FQDN must terminate at the same IP address (A record). (You can find details in | RFC 1912 and the Wikipedia article on FcrDNS.) In particular, we recommend choosing a hostname that indicates its usage as a mail server (eg. mail.example.com) and to ensure the host's | domain leads to a website providing full contact details.(...)"

But somehow not able to figure out issue behind rejection of mail. Request your support here.