What to do If I Forgot My Yahoo! Username – Recover Yahoo! Mail Username
Your Yahoo! ID is usually the first part of your Yahoo! Mail address. For example, if your Yahoo! Mail address is email@example.com, your Yahoo! ID is “person.”
If you can’t remember your Yahoo! ID, you can try logging in using any of your other Yahoo! profiles, as long as they’re part of the same Yahoo! account.
For example, if you know you’ve got a profile called “chatperson” that you use in Yahoo! Chat, you could log in to Yahoo! Mail using your “chatperson” profile and your password. This logs you in to your Yahoo! Mail account, and you’ll then be able to see your Yahoo! Mail address at the top of your Inbox.
You can also use the Sign-In Problems Wizard page to find your Yahoo! ID. To get there, click the “Forget your ID or password?” link on the log-in page. You’ll be asked for your birth date and ZIP code or country. We will then request for the alternate email address that you provided when you set up the account, so we can send the information there.
Email : firstname.lastname@example.org