how can we use yahoo forgot password method in c#

There is also not a case in which you can see if that email account exists or not. Instead, you need to test if the email is bouncing or not. A bounced email is the email that cannot be delivered to the email address later. 

 

In Gmail, you can get an email telling you that the email was bounced. However, this is also a standard based method to determine if an email was delivered or not. You can get to see about it, below:

 

http://stackoverflow.com/questions/7393921/detecting-bounced-messages

http://stackoverflow.com/questions/9104404/detecting-bounced-messages-by-return-path-header

 

These would allow you to determine if the email address is valid or not.  

ya its good..but my requirment is to check an email address is existing or not,not only yahoo it includes gmail,hotmail,yahoo etc..

Forgot passport method works on "existing" emails. If you do not have an email account, or you do not know it, you cannot use that method. The behavior is undefined. 

 

That is why, it won't be possible to know if the user is entering their own existing email address. For that sake my recommendation is something different.