About domain extensions in Yahoo Mail
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.