There is no SMTP client on NETCF like there is on the full .NET framework. YOu have two options. One is to use POOM (Pocket Outlook Object Model) using the COM Interop layer provided by NETCF and the other is to just implement your own SMTP layer over TCP/IP.
Send E-mail - SMTP Autentication
C.Z.
For the first refer Steven's blog at:
http://blogs.msdn.com/stevenpr/archive/2004/08/05/209390.aspx
For the second:
Here is a sample code ...
http://blog.opennetcf.org/ayakhnin/PermaLink.aspx guid=53260a37-c41c-42df-9a6d-6a61bf5c5317
And here is where the code was inspired from ...
http://www.eggheadcafe.com/articles/20030316.asp
HTH