We have a content management system that is running based on the IIS 6.0. We never had problem to open .msg file from the browser until one day I re-configured the IIS server on the same server. "Http error 404 - File or Directory not found" message was being displayed every time when I tried to open a .msg file from the browser. I immediately filed a service call to their support group because usually they could found the problem really quick if it is something related to their product. I did metioned this problem started happening right after I re-configured the IIS server. However, the tech support seemed to have his own idea on how to troubleshoot. He started looking at the settings on the File-Type all the way to web server, and finally asking to remove the ISAPI dll file to see if that was casuing the problem. Just like what I expected, that was not the case. Finally, I thought I should seriously look at this by myself as it seems to me that the problem is not only related on their product but on the web server as well. After I ran a couple of searches on Google, here is what I found, which is exactly what the problem is, particulaly for IIS 6.0. The solution resolved the problem completely.
What is casuing the problem?
Earlier versions of IIS include a wildcard character MIME mapping, which permits IIS to serve any file regardless of its extension. IIS 6.0 does not include this wildcard character MIME mapping and does not serve any type of extension that is not defined at the MimeMap node in the IIS metabase.
What is the solution
To define a MIME type for a specific extension, follow these steps:
1.Open the IIS Microsoft Management Console (MMC), right-click the local computer name, and then click Properties.
2.Click MIME Types.
3.Click New.
4.In the Extension box, type the file name extension that you want (for example, .msg). You can also put "*" here If you want to serve all files regardless of the file name extension. However, it is not recommended.
5.In the MIME Type box, type application/octet-stream.
6.Apply the new settings. IIS now serves files with the extension that you added. In this example, IIS now serves files with the .msg extension.
IIS 6.0 Does Not Serve Unknown MIME Types
notes: this solution is not particularly for opening .msg file. It should work for any file that has the extension name that is not defined as MIME type on the IIS 6.0.