I think you might be using the wrong smtpserver or wrong port. Please check 
with yahoo. 


Yahoo pop3 and smtp works only if you have a Yahoo! Mail Plus account. 

Here are some of the data below

Here are the basic server settings for Yahoo! Mail:
Incoming Mail (POP3) Server:	plus.pop.mail.yahoo.com (Use SSL, port: 995)
Outgoing Mail (SMTP) Server:	plus.smtp.mail.yahoo.com (Use SSL, port: 465, use authentication)
Account Name/Login Name:	Your Yahoo! Mail ID (your email address without the "@yahoo.com")
Email Address:	Your Yahoo! Mail address (e.g., user@yahoo.com)
Password:	Your Yahoo! Mail password

Here's the the link in case u have a yahoo mail plus connection 
http://muruganad.com/ASP.NET/ASP_.NET_Send_Mail_from_YMail_or_Yahoo_Mail_using_MailMessage_SmtpMail.html