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" http://support.microsoft.com/kb/942076
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 + http://blogs.iis.net/thomad/archive/2007/12/17/iis7-rejecting-urls-containing.aspx