I had a bit of trouble decyphering the MSDN article below:
Using a username and password with the new .net mailer is actually quite easy (C#):
SmtpClient mailer = new SmtpClient();
mailer.Host = "mail.youroutgoingsmtpserver.com";
mailer.Credentials = new System.Net.NetworkCredential("yourusername", "yourpassword");
mailer.Send("email@example.com", "firstname.lastname@example.org", "Subject line", "Main email text body");
As a sidenote, I was trying to send from an smtp server that was running on the same system as the site I was working on. That server uses the no-ip service to redirect the domain name to the dynamic IP the server sits at. The problem I ran into was that hotmail totally rejected the sent messages on receipt. I could send them ok but they just never came through on the other end, at the hotmail recipient. It took alot of digging around for me to find out that it rejects mail sent from certain dynamic IP hosts such as no-ip. If you want to send it then you will have to use an smtp server that is NOT with one of these hosts.