Dear,
There must be typo error in the email you wrote
i received the similar error and i resolved it by correcting the email.
just review your email it would solve the problem

Hi,

the same happens to me. And there's no typo in my email address.
I'd be very grateful if this issue was fixed.

Thanks,
meiser

Hello,

how can we get some attention on this issue?

Regards,
meiser

I'd suggest contacting the GitHub Support: https://support.github.com/contact/account

Feedback from Github support:

I've had a look into this for you, and unfortunately the @gmx-topmail.de domain is one which we have experienced an increased number of issues with, and as such it is no longer possible to verify email addresses from that domain.

Whilst I know it may be inconvenient, I'm afraid you will need to add another email from a domain which you are able to successfully verify.