Dave Wanta (of kbAlertz.com and 123aspx.com) has put together some really great info on the System.Web.Mail namespace at SystemWebMail.com. This FAQ is one of the most amazing & complete .NET resources I've seen in a while. Really awesome stuff.
You can find all kinds of undocumented stuff here. For example, many might say that you cannot send authenticated SMTP via the System.Web.Mail namespace. But Dave says phooey and gives a clear example in his FAQ. How about adding custom headers, like a Reply-To header? He's got an example for that too and another for adding your own custom headers.
One unrelated thing about the site that was cool is it's great use or URL rewriting. I love that stuff. But you know, it's guys like Dave that make me feel like I don't contribute enough to the online programming community.