To send mail using System.Net.Mail, you need to configure your SMTP service in your application's web.config file using these values for mailSettings:
<system.net><mailSettings><smtp from="your email address"><network host="relay-hosting.secureserver.net" port="25" /></smtp></mailSettings></system.net>
The network rule's value will be used when you instantiate an SmtpClient in your code.
You can then use code similar to this to send email from your application: