a. System.Web.Mail is not supported under version 3.5 of the Framework.
b. System.Web.Mail is depredcated under version 3.5 of the Framework, and it is officially recommended that System.Net.Mail be used.
c. System.Web.Mail is the preferred solution when using IIS hosted applications.
d. There are no functional differences; the items were moved to a new namespace to better reflect their applicability.