For ASP.net 1.0 and the older System.Web.Mail see:

http://knowledge.3essentials.com/web-hosting/article/1048/SMTP-AUTH-in-ASP.nets-System.Web.Mail.html

For ASP.NET 2.0 includes much richer Email API support within the System.Net.Mail code namespace.  Here is a simple snippet of how to send an email message from “me@mydomain.com” to multiple email recipients (note that the To a CC properties are collections and so can handle multiple address targets):

MailMessage message = new MailMessage();

message.From = new MailAddress("me@mydomain.com");
 
message.To.Add(new MailAddress("recipient1@yourdomain.com"));
message.To.Add(new MailAddress("recipient2@yourdomain.com"));
message.To.Add(new MailAddress("recipient3@yourdomain.com"));
 
message.CC.Add(new MailAddress("carboncopy@yourdomain.com"));
message.Subject = "This is my subject";
message.Body = "This is the body of my email";
 
SmtpClient client = new SmtpClient();
client.Send(message);
 
System.Net.Mail reads SMTP configuration data out of the standard .NET configuration system (so for ASP.NET applications you’d configure this in your application’s web.config file).  Here is an example of how to configure it:
 
  <system.net>
    <mailSettings>
      <smtp from="me@mydomain.com">
        <network host="mail.mydomain.com" port="25" userName="me@mydomain.com" password="mypassword" />
      </smtp>
    </mailSettings>
  </system.net>