One of the method to know the email address which I came across has been listed below which you can try yourself.
1. Open the profile of the person whose email address you wish to know. It can be anyone in the Orkut network.
2. Click on "Ignore User"
3. Open GTalk and sign in with your ID
4. Click on Settings and select "Blocked"
5. You will find the email address of the person whom you blocked on Orkut.
You can see the sceenshots of the same at http://vikrantweb.blogspot.com/2007/03/orkut-your