>First thought: Interop often used to describe Dotnet#COM interaction, perhaps only the COM wrapping of those dll's is gone ?
>Second thought: Dotnet loading from other locations (less danger on a server as less is installed/tested, but still a possibility

Those two files are present in a separate directory. There are no relation between the two.

>afterthought: check loaded dlls with Process Monitor from Systernal or try dependency walking

I checked with that utility. I found one copy of that file under a previous compiled of the Web site. This was a hidden directory under the .NET Framework 64 bits. It had no effect. I did remove it, restarted the IIS service and that still works.

When I checked with the utility, I also found no instance of that in memory.

The Framework and the Web site also do not have any reference to the Interop .NET library.

I however wanted to get rid of that since a very long time. I found this great utility http://www.independentsoft.de MSG .NET.

I am doing the final tests and so far this works great. This is only one DLL to drop in the Bin directory. That's it.

I already built a version of my MSG reader to support that utility. The price is also competitive. I have found some freeware we can use with full source code spreading over thousands of lines. That will not justify the investment in time to go through all that, and in this case, I prefer to have something that evolves as this is a product which seems to be well supported.