Did you get your IP successfully whitelisted? I’m having the same issue. Apparently, t-online.de puts entire IP blocks from hosting companies (like linode) and ISPs (like TWC) in their black lists by default.
I emailed the contact address and was given a few key bits of information. First, was that hosts that are de facto anonymous to third parties are not qualified for delisting. I’m trying to figure out what this means. Does this refer to making the domain registration information public?
The second thing was from their docs at https://postmaster.t-online.de/index.en.html, Chapter 4.1, which state, among other things, that I must “ensure the host’s domain leads to a website providing full contact details.” Has anyone setup a Webpage like this before?