Update
Please navigate to this post for the working trick for this hack
Note
Hack listed below does not work anymore. Orkut has changed the design of friends page. Please see this post for working version of this hack 
This hack will allow you to find email address of any orkut member even if he or she has not mentioned it in his or her profile respectively.
» Instructions
- Navigate to the profile of the member whose email address you want to know.
- Add that person as your friend.
- Now, Navigate to the friends page. You need not wait for an approval.
- Goto Open Friend Request on top left corner. (refer to screenshot above)
- Click on edit on the upper right hand side corner of the box representing the member you just added as friend.
- You will find his or her email address there.
- Now delete the person from your friend’s list by clicking delete button given on the same page(optional).
» Notes and Updates
This hack is not working anymore. Please enjoy other ‘working’ hacks until we dig out an alternate way for the same.
Gaurav Dua – Orkut Guru – 546 Points 
22, Webmaster and Businessman. Based in Jammu, J&K – India
Gaurav has written 514 Articles, posted 323 Comments.
how to know the email of an account of orkut without being its friend?