Dave Wanta, my old friend from way back, has created a resource site for System.Net.Mail called www.systemnetmail.com.  Now, this is not just a rehash of the MSDN doc's.  Dave does a wonderful job of explaining how to use this namespace (samples, dependencies, infrastructure requirements) and also explains the strengths and weaknesses - like BCC's are not so blind.

I also recommend checking out Dave's company site, AdvancedIntellect.  It has a great look and it's all .NET.  You also see a list of interesting .NET products - that you'll hardly find anywhere else.

Dave is also the originator of 123aspx and kbAlertz.  He was also one of the early educational ASP.NET resources for me way back when .NET was coming out.  He, Dan Fox, and I co-wrote the original Atomic training series for Quilogy in a past life time.  I can tell you that Dave is the man and someone that I truly respect!