For users that register a ymail.com or a new rocketmail.com account, you're Yahoo! ID will be your full email address which contains the '@' character. If you have registered a yahoo.com account, your ID will not contain the '@' character.

Partner Domains: Yahoo! has relationships with some partners where the Yahoo! ID is also an email address in a domain belonging to a partner. For example, customers have " Yahoo! IDs" ending in "@yahoo.com" that they need to type at login.