The guy from KBAlertz, Dave Wanta, has a site devoted to the System.Web.Mail namespace and adds lots of value to the standard MSDN documentation (this doesn't just reproduce the docs the way some annoying sites do -- I won't even bother giving those links and boosting their page ranks!): http://www.SystemWebMail.com/
I've turned to the site on a couple of occasions, particularly to copy and paste the boilerplate “send an email using server authentication” code:
MailMessage mail = new MailMessage() ;
// ... set regular properties here ...
mail.Fields.Add( "http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1" ) ;
mail.Fields.Add( "http://schemas.microsoft.com/cdo/configuration/sendusername", "my_username_here" ) ;
mail.Fields.Add( "http://schemas.microsoft.com/cdo/configuration/sendpassword", "super_secret" ) ;
SmtpMail.Send( mail ) ;
Who wants to remember all those //schemas.microsoft configuration strings anyway?
Yes, we've rolled this into our own Mail component extending MailMessage, but from time-to-time www.SystemWebMail.com comes in real handy. Give Dave Wanta's site a look!