Error subscribing new mailing list.

Hi,

I have tried to subscribe the new mailing list <[email protected]>
via <https://admin.gmane.io/>. This results in the error message

--8<---------------cut here---------------start------------->8---
Request adding this list to the Gmane archive...Your address
[email protected] is invalid: Initial greeting: 554-gmx.net
(mxgmx116) Nemesis ESMTP Service not available 554-No SMTP service
554-Bad DNS PTR resource record. 554 For explanation visit
https://www.gmx.net/mail/senderguidelines?ip=116.202.254.214&c=rdns
Process check connection broken by remote peer
--8<---------------cut here---------------end--------------->8---

What's going wrong? I'm pretty sure that messages reach me via
<[email protected]> ...

Best regards, Michael.