You can modify the IIS SMTP server welcome message on Windows Server 2003 or Windows 2000 to display a custom message by adding a ConnectResponse Metabase Property.  

When you connect to a mail server using telnet you will receive a message telling you some basic information about the mail server, including FQDN (fully qualified domain name) of the server, the protocol, server version and the date, time and time zone.

The following is an example of such a welcome message:
220 Localhost Microsoft ESMTP MAIL Service, Version: 6.5.7226.0 ready at Sun, 04 Nov 07 10:42:33 +0100

We can change this message to hide the server type and version number, or to put a unique identifier in there to help you remember which server you are connected to. Why hide the server type and version number? Well it allows a hacker to find out what type of server you are running, changing the banner to look like the banner for a different type of server, Sendmail for example, would have them attacking Sendmail exploits which should have no effect on your Windows Server 2003 IIS servers.

This procedure explains how to enable the Enable Direct Metabase Edit feature in IIS Manager. You must perform this procedure in order to edit the MetaBase.xml file in the IIS 6.0 metabase directly when IIS is running; otherwise the file remains open and locked when IIS is running.

1. In IIS Manager, right-click the server object, and then click Properties.
2. Select the Enable Direct Metabase Edit check box. 

Now open the MetaBase.xml file default located in C:\WINDOW\system32\inetsrv\ with a text editor of your choice and search for this string: IIsSmtpService

Right below add a new Metabase Property called ConnectResponse and give it a value containing the new welcome message and save the file, example:
ConnectResponse="[-- SPAM not welcome here! --]"

When we now use telnet to connect to this SMTP server it shows this message:
220 Localhost [-- SPAM not welcome here! --] Sun, 04 Nov 07 10:49:23 +0100

As the majority should be using Windows Server 2003 by now i don`t go into detail on how to accomplish this on Windows 2003, instructions for IIS 5.0 on Windows 2000 can be found here

note: You must restart the IIS Admin service and all its dependent services, including the SMTP service, to save the changes. The SMTP service is designed to obtain changes to the system configuration automatically, without requiring a restart. But some modifications, such as changing the SMTP banner, might require a restart. 

UPDATE:
I crafted up a little tool to automate this process, download it below.