Identify your domain extension in Yahoo Mail
The domain extension is the ".com" part of your email address. While most Yahoo accounts end in "@yahoo.com", some older accounts may have a different extension depending on the location 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 when giving out your email address. If an email is sent to the wrong domain, it won't be delivered.
Find out what your Yahoo domain is in Yahoo Mail settings.