My provider rejects the authentication email

When I try to post to

    gmane.comp.graphics.darktable.user

then my newsreaders, GNUS and Thunderbird, report a cryptic error
message:

“441 You need a valid email address to post: Invalid response from SMTP
server mail.inka.de: 550-Verification failed for <[email protected]>”

So I contacted the admin of mail.inka.de. He said, roughly translated by
me from German:

“They also ask (apparently on POST) via SMTP whether your email address
exists. But it doesn’t make sense to do that with an email address that
itself is not deliverable:

    2020-07-13 15:04:30 H=static.214.254.202.116.clients.your-server.de
    (ciao.gmane.io) [116.202.254.214] F=<[email protected]> rejected
    RCPT <[email protected]>: Sender verify failed

    > exim -bt [email protected]
    [email protected] is undeliverable: Unrouteable address

That triggers our Sender Verify, which says: go away.”

*Any idea what to do? Is there another way to verify my email address?*

(using another email address is not an option)