SMTP and Windows

Collaboration Data Objects (CDO)

Microsoft has had quite a history of supporting Internet messaging on various platforms. To understand the problems associated with .NET mail support, you need to know the history of Microsoft mail support. It started with the Microsoft Exchange 5 mail system and the Active Messaging library (OLEMSG32.DLL). This library allowed programmers to write code that utilized the Exchange messaging system to send messages to individual user mailboxes, as well as to other remote Exchange hosts in the Exchange network.

With the release of Exchange 5.5, a new messaging system was introduced, Collaboration Data Objects (CDO). Unlike Active Messaging, CDO used the Message Application Program Interface (MAPI) standard to provide programmers a simplified way to send messages. CDO survived for many years, with many upgrades, including versions 1.1, 1.2, and 1.2.1. With the Microsoft NT Server platform came the CDO NT Server library (CDONTS). This library broke away from the MAPI standards used in the other CDO versions and started using Internet standards, such as SMTP, to transfer messages to remote hosts.

The current version of CDO (CDO 2, also known as CDO for Windows 2000) was released with the Windows 2000 platform and is used on the Windows XP platform as well. It expands the work done with the CDONTS library in using Internet standards to provide mail and news service libraries to programmers. Other new items in CDO 2 are the ability to process multipart mail attachments and various mail protocol events. These features help programmers create full-featured mail server code with minimal effort. CDO 2 greatly expanded the Internet mail functionality provided to programmers and is the library that .NET uses to support its Internet mail features.

In Windows 2000 and XP, the CDO 2 library is the file CDOSYS.DLL. This file must be present for all .NET mail library functions to work. Because CDO 2 is completely different from the CDO 1.x versions, both libraries can be present on the same machine if necessary. Unfortunately, the CDO 2 library is not backward compatible with older Windows platforms.

SMTP Mail Service

Both Windows 2000 and XP provide a basic SMTP server service that supports both sending and receiving mail messages using the SMTP protocol. This functionality is included as part of the Internet Information Services (IIS) package. The .NET mail classes can utilize the IIS SMTP server to send outgoing messages directly to remote mail servers.

Windows XP systems include IIS version 5.1, which among other things includes the SMTP service. Before you can use the SMTP service, it must be configured to work properly in your Internet mail environment. Configuration is done from the Computer Management window.

Here are the steps to access the Default SMTP Virtual Server Properties window:

1. Right-click the My Computer item in the Start menu, and select the Manage menu item.

2. When the Computer Management window appears, expand Services and Applications and then Internet Information Services.

3. Right-click Default SMTP Virtual Server and select Properties.

The Properties window is where you’ll configure the settings for the SMTP service.

In the General tab, select the network interface(s) that should allow incoming SMTP connections and the number of concurrent SMTP connections allowed. Also, you can enable/disable logging for the SMTP service.

In the Delivery tab (Figure), configure delivery retry times. The Advanced button reveals settings for a smart host for the mail server. This is important if your ISP does not allow you to directly communicate via SMTP to remote hosts. Often you must use your ISP mail server as a mail relay to reach remote mail servers. To do this, you must enter the ISP mail server address as the smart host, and the SMTP service will forward all outgoing messages through that mail server.

Figure: The Default SMTP Virtual Server Properties dialog box is where you configure SMTP settings.