Email Issue from iPhone

As of fairly recent, I can’t send emails to a specific person from my iPhone anymore. No problems from my Mac. I’ve even tried two different mail apps (Mail and Google Mail).

I also ensured that I was whitelisted/added to the address book of the recipient... again messages from my laptop arrive just fine.

I get bounce back messages as follows: “Message blocked

Your message to myfriendsname@gmx.de has been blocked. See technical details below for more information.

The response from the remote server was:

554 Transaction failed Reject due to policy restrictions. For explanation visit https://www.gmx.net/mail/senderguidelines

I don’t know what settings I can reset to be able to send emails again. Help please!!