Let's Encrypt false positive 403 response when used with certain e-mail addresses

We found that Let's Encrypt delivers false "403" responses when trying to create a certificate, although the web server delivers the token with a 200 OK code, the token can be retrieved by a browser, DNS is set correctly and propagated all through the internet. Nevertheless for one customer it was impossible to create a Let's Encrypt certificate. We always got the "403" response and the notice, that the token does not match the challenge.

This error message is wrong. The true reason for the...

Let's Encrypt false positive 403 response when used with certain e-mail addresses