@stellated
There's a document where they list their requirements but in the end it's a whitelist. They deny incoming mails from every IP which is not on their list. If you contact them to basically get on the list, they manually check your smtp host if you meet their requirements and whitelist you.

Here's their postmaster Accepable Use Policy document: https://postmaster.t-online.de/index.en.html
@Bebef @kkarhan @badkeys