Unless you know their exact user name you are going to have a problem finding them. It also depends on the poker room you are playing at.

Many of them keep logs of your activity on the hard disk. If your site does that, you might be able to see the user names of people you played against.

That is probably the only way to check since you have the user id and the address. their is a site in yahoo where you can look up user ids and email address but you have to have the real name maybe they will play again tonite or just loo through all the tables

Sorry, unless you know their user ID, or entire e-mail address, I believe checking all game rooms is your only option ,, Goodluck !!!!