CDONTS, CDOSYS, System.Web.Mail & System.Net.Mail
Microsoft Windows Server 2003 does not install Collaboration Data Objects (CDO) for NTS (CDONTS). Therefore, applications that use CDONTS do not function on a Windows Server 2003-based computer.
Windows Server 2003 provides improved alternatives to CDONTS. To make applications that use CDONTS function on a Windows Server 2003-based computer, update existing CDONTS applications to use one of the following technologies:
CDO for Windows 2000
CDO for Windows 2000 provides a broader functionality set than CDONTS. This functionality includes the ability to send messages through a remote Simple Mail Transfer Protocol (SMTP) server. You can use CDO for Windows 2000 (CDOSYS) in applications that are not Microsoft .NET Framework-based.
For example, Microsoft Visual Basic 6.0 and Microsoft ASP are not based in the .NET Framework. You can also use CDOSYS in .NET Framework applications.
The System.Web.Mail namespace provides a managed wrapper for CDOSYS. The System.Web.Mail namespace is only available in .NET Framework applications.
When we talk about .Net framework, we can use the System.Net.Mail namespace. It provides way to send electronic mail to a Simple Mail Transfer Protocol (SMTP) server for delivery.
FYI: System.Net.Mail Vs System.Web.Mail
In .NET 1.1, only the System.Web.Mail was available. This implementation needs to be done in quite different way. For example, attachment could only be added from files, not from Streams. So, this can be still used under .Net Framework 1.0 and 1.1, but going forward it’s “deprecated” or “Obsolete”; so it’s recommended that to make use of System.Net.Mail.
The .NET 2.0 implementation is System.Net.Mail and is much more flexible and has a richer feature set.