Pff. That is nothing. Try to run your own mailserver and deliver a mail to a t-online.de address (T Mobile Germany).

They basically only accept pre-approved providers. If your have your own domain and infrastructure you have to petition them to whitelist you. Totally insane.

If you can read German, this guy who runs a shop decided to block himself all of t-online emails since they basically run email out of specification. https://blog.rolandmoriz.de/2020/09/21/t-online-blockiert-ma...