As of 2020-12-28, t-online does not accept mails from me with: 554 IP=184.108.40.206 - A problem occurred. (Ask your postmaster for help or to contact firstname.lastname@example.org to clarify.) (BL)
Communication with email@example.com brought up that t-online has apparently setup own rules which are not backed by RFCs. The requirements I got:
request: Have a system with a name like 'mail.<domain>' deliver mail to t-online.
comment: That makes no sense. google would then start to request me to deliver from 'foobar.<domain>', so what should I do then? The RFC allow me to deliver plainly from <domain>. I have also all DNS things like dmarc in order, I am reacting to mails to postmaster@domain and so on.
request: Your whois record does not have your full contact details (that's because of new data protection laws). Providing these details via https from the same domain would be acceptable.
comment: I already have my name and various ways to reach me described on https://fluxcoil.net . My site is not commercial, I do not need an “impressum” as per German law. This is an arbitrary request from t-online. If such a request is valid, it should be discussed in the community and find its way into RFCs. Otherwise, everybody on the internet can start to setup such “own rules”.
⇒ For now, I send mails to t-online from a different mail account, and notify the recipients that their provider is “special”.