.net: System.Web.Mail vs System.Net.Mail
Solution 1:
System.Web.Mail is not a full .NET native implementation of the SMTP protocol. Instead, it uses the pre-existing COM functionality in CDONTS. System.Net.Mail, in contrast, is a fully managed implementation of an SMTP client.
I've had far fewer problems with System.Net.Mail as it avoids COM hell.
Solution 2:
Biggest issue with System.Net.Mail is that it has no support for Implicit SSL. Use System.Web.Mail until you don't have a need for Implicit SSL support.
Solution 3:
here are 2 sites that provide documentation and samples for both
http://www.systemwebmail.com/
http://www.systemnetmail.com/