It can be pretty tricky getting the smtp configured correctly especially if using Yahoo or Google to send the mail. I know I struggled with it for a while before I got the right combination for Yahoo. Their documentation is sometimes incorrect in some places it indicates one port and in others they indicate a different port. My settings that work for Yahoo are like this:
<add key="SMTPServer" value="smtp.bizmail.yahoo.com" />
<add key="SMTPRequiresAuthentication" value="true" />
<add key="SMTPPort" value="587" />
<add key="SMTPUser" value="foo@foo.com" />
<add key="SMTPPassword" value="foo123" />
<add key="SMTPUseSsl" value="false" />
actual user and password changed to protect the innocent.
Yahoo instructions say to use SSL but it only worked for me when it was set to false. So sometimes you have try settings that don't seem right according to the documentation.
Hope it helps,
Joe