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I was enabling VCard downloads on the NMSU phonebook when I noticed that it wasn't working in IE. "Internet Explorer was not able to open this Internet site. The requested site is either unavailable or cannot be found. Please try again later." Of course it worked fine in FF, Opera, Safari, Camino, and just about any other browser available.

Well thats what you get for not testing on all browser you say? Actually it worked fine in IE on our development server. Same code two different boxes. It was driving me nuts, what could be the problem? Is it something to do with the headers that the page is sending? Using FF and the FireBug extension I ruled out that possibility. Both servers were sending identical headers. The only difference is the producation server had SSL while the development did not.

It didn't make sense.

Then, thanks to Google, I stumbled upon the answer:

In order for Internet Explorer to open documents in Office (or any out-of-process, ActiveX document server), Internet Explorer must save the file to the local cache directory and ask the associated application to load the file by using IPersistFile::Load. If the file is not stored to disk, this operation fails.

When Internet Explorer communicates with a secure Web site through SSL, Internet Explorer enforces any no-cache request. If the header or headers are present, Internet Explorer does not cache the file. Consequently, Office cannot open the file.

Oh, that makes sense.

When Outlook tried to open the VCard file, it couldn't find it because it was not stored on the disk.