Merlynx,
Before you can import a namespace you need to reference the assembly.
Most (all?) of the classes in the System.Web.Mail namespace are located in
the System.Web.dll assembly.
You will need to include System.Web.dll in your references when you compile
your program.
To find out the assembly that needs to be referenced check the Assembly
entry in the Requirements section of the overview page for the Class
(Struct, Interface, Enum) on MSDN.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/
frlrfSystemWebMailMailMessageClassTopic.asp
Hope this helps
Jay