How about using
<add key="SmtpClientEnableSsl" value="1" />
I wanted to post it as comment but it had tags and automatically was removed.
[After you comment]
These people had the same problem as you :
http://stackoverflow.com/questions/777607/the-remote-certificate-is-invalid-according-to-the-validation-procedure-ple
http://stackoverflow.com/questions/7856475/the-remote-certificate-is-invalid-according-to-the-validation-procedure
http://blog.jameshiggs.com/2008/05/01/c-how-to-accept-an-invalid-ssl-certificate-programmatically/