Most Yahoo accounts end in “@yahoo.com”. The domain extension is the “.com” part. Some older accounts have a different extension depending on the location and language where they were created. You can no longer create accounts with Yahoo domains other than “@yahoo.com”.
If you have a Yahoo ID with a different local domain, like “@yahoo.co.uk” or “@yahoo.fr”, make sure you include the correct domain extension–the “.co.uk” or “.fr” part–when giving out your email address. If an email is sent to the wrong domain, it won’t be delivered. To ask what is the issue you can contact yahoo customer service