Disclaimer: This works ONLY for newly created accounts and NOT pre-existing ones.
Step by step:
1.)Register an account at my.yahoo.com, and UNCHECK the create email automatically.After registering click MY YAHOO! which is on top of Registration: Please Activate Your Account
2.)Go to cn.mail.yahoo.com. Do not log out of the my.yahoo.com account.
3.)The interface is in chinese. Just enter your name and surname in the field and click activate.
4.)Now, logout of the account.
5.)You will be redirected to the login page. Click the orange button that is closest to the login box.
6.)Follow the intrustions and install the chinese messenger.
7.)After installing, go back to cn.mail.yahoo.com and click the button again.
8.)Login to your account.
9.)Congrats, you have a 1 Gb mailbox.
I havent tried it personally ,but i got it from someother forums. It has positive replies, so pretty sure it works 
Give it a try 
