Signing in with a Ymail or Rocketmail account
If you registered for a Yahoo! account with either a ymail or rocketmail email address, be sure to include "@ymail.com" or "@rocketmail.com" when you sign in to your Yahoo! account.
Example: If your ID is "johnsmith75", and you have a "ymail.com" email address, you will need to enter "firstname.lastname@example.org" to sign in to your Yahoo! account.