I ran into this same issue and then discovered the following KB:
Error message when you visit a Web site that is hosted on IIS 7.0: "HTTP Error 404.11 – URL_DOUBLE_ESCAPED"
Setting the allowDoubleEscaping="true" did resolve my problem but this approach slightly compromises the server security by leaving it open to “Double Encoding” attacks.
The cause of the problem was the “+” sign in the name of some of the .eml files. In fact any file that has a “+” in the file name + is hosted on IIS7 + does not have the allowDoubleEscaping set to true will give you the HTTP Error 404.11.
IIS7 rejecting URLs containing +