Here you will find a selection of articles about SMTP and .net fx.
If you have issues trying to send e-mails using .net, I would (you can find this info at the link provided):
1. First check if the problem is located on your .net code, or if it's a network issue (permissions, configurations...), you can try to send an e-mail using telnet.
2. If you are using .net fx 1.1 go for System.Web.Mail Oh My !
3. If you are using .net fx 2.0 go for System.Net.Mail Oh My !
4. If you want to build something more complex (e.g. embedd images on your e-mail), you can do it so using .net fx 2.0