If I have only the user name for an email address, is there a cross reference online that I can use to get the host name also? Thank You. That is, I need the Domain name after the @.

No, that is impossible. For example if the username is "Jim12" then that username can be used millions of times, each with different a domain name, and still be considered a unique email address.

Sorry no,

That's like asking, how many Fred's live on 12th street.