CB - Unable to register "This email recieves no email"

Hello, we are using CB1.8 with J1.5.26.

Sometimes a user is not able to register. He gets an ajax verification error based on the email like "Diese E-Mail-Adresse empfängt keine Nachrichten: Bitte prüfen, sie ist zur Anmeldung erforderlich." (This email recieves no messages: Please verify, its needed for registration).

The email is valid and will recieve mails. I have tried to set the ajax verification off, but it still verifies the email. This happens also on the backend.

What typ of verification fails and how to fix this issue?
We are moving to J25 within the next month, so I dont want upgrade to the last CB version within J15.

Thanks for support & br
Wolfgang

The ajax verification is optional. It's there for the users convenience. You can disable it on frontend from CB > Configuration > Registration, but it'll always function in backend. They can continue with registration regardless of what verification says. It trys to verify with a MX Lookup and some mail servers don't respond to lookups (gmail for example doesn't).