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SMTP: Then and Now

SMTP in Exchange Server seems to have become more complicated. Jerry Cochran explains why.

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Why is SMTP in Exchange 2000 Server so difficult to figure out? SMTP in Exchange Server 5.5 and earlier versions is relatively easy to understand and set up: For the most part, you have just one SMTP service, called the Internet Mail Service (IMS) or Internet Mail Connector (IMC) for Exchange Server 4.0. Configuration of this service is rather straightforward, and the service usually does exactly what you tell it to do. Exchange 2000, however, greatly complicates the process, using SMTP virtual servers, SMTP connectors, and a new Routing service. In fact, SMTP configuration is one of the leading Exchange 2000 topics for Microsoft Product Support Services (PSS—see Microsoft's Exchange 2000 Server Support Center site at;en-us;exch2k ). Therefore, I want to take a moment to look at how SMTP has evolved in Exchange 2000.

First, be aware that comparing SMTP in Exchange 2000 with SMTP in earlier Exchange versions isn't exactly an apples-to-apples comparison. In Exchange 5.5 and earlier versions, SMTP is simply an add-on connectivity protocol option; X.400 is the core Message Transfer Agent (MTA) protocol. Exchange 2000 reverses this arrangement (sort of): SMTP is the core MTA protocol and X.400 support is the add-on connector option. Part of the reason that SMTP is so easy to understand in Exchange 5.5 and earlier versions is because it isn't the core delivery mechanism. Now that Exchange 2000 relies on SMTP as its core transport and delivery mechanism—in Exchange 2000, all message delivery is SMTP-based by default—figuring out SMTP's role and configuration options is much more daunting.

SMTP in Exchange 2000 is based on three key components or objects: SMTP connectors, SMTP virtual servers, and the Routing service. An SMTP connector has the same role as the IMS in Exchange 5.5 and earlier versions. The reasons for using SMTP connectors in Exchange 2000 (e.g., SMTP connectivity, ETRN/TURN and HELO/EHLO support, SMTP security settings, message delivery and content restrictions, address spaces) are the same as the reasons for using the IMS in Exchange 5.5. When configuring an SMTP connector in Exchange 2000, you'll recognize some of the same settings and configuration options that you find when configuring the IMS in earlier Exchange versions. You use SMTP connectors for specific outbound connectivity within your organizations and to the outside world (i.e., the Internet). By default, you don't need any configured SMTP connectors for email to flow around your Exchange 2000 organization. That's where SMTP virtual servers and the Routing service come in.

In Exchange 2000, Microsoft IIS SMTP virtual servers and the Exchange 2000 Routing service replace the core X.400-based MTA found in Exchange 5.5 (and earlier versions). Exchange 2000 no longer hosts the SMTP protocol but relies on IIS to provide this functionality through SMTP virtual servers. These virtual servers are dedicated to providing SMTP service to Exchange (through TCP port 25). Together with the Routing service—which is a routing, categorization, and resolving service—SMTP virtual servers provide the core message delivery system for Exchange 2000.

So, how does all this apply to how we run our Exchange 2000 deployments? When you want to manage how messages flow into your Exchange 2000 organization, concentrate on SMTP virtual servers, which receive messages (both external and internal), and the Routing service, which handles how to get messages where they need to go. When you want to send messages to another organization or to the Internet, concentrate on SMTP connectors, which are hosted by SMTP virtual servers and control this connectivity. (You can also use SMTP connectors to link Exchange 2000 Routing Groups, but that process is outside the scope of this discussion.)

At first look, SMTP support in Exchange 2000 might seem a bit confusing in comparison to the simple and straightforward approach you're used to in earlier versions of Exchange. But although Exchange 2000's SMTP configuration and management might be more complex, the power of SMTP in Exchange 2000 brings many advantages in terms of functionality, manageability, and reliability. If you're planning a migration to Exchange 2000 or are in the middle of one, it might be worth your time to take a close look at the changes in SMTP support.

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