Having a problem with specific file attachments on OWA 2003 where users get an authentication prompt when attempting to open msproject attachments (.mpp). This prompt only occurs with this attachment type. I've verified that iislockdown/urlscan was NOT run in this environment, nor is the mpp file type included in any of the native OWA attachment blocking registry keys. Structurally, this is a somewhat complex environment - Ex2k3 Front end server to an ex2k3 back end server, and further front-ended by an ISA server. If I attempt to access OWA on the mailbox server itself, I can open the mpp attachments without any prompts. However, if I focus my URL on either the front end OWA server, or the isa server, I get the authentication prompt. Note that I can click on cancel in the authentication prompt and the attachment will actually open. If it was an authentication or delagation issue between the front end/back end server, then I can't understand why it would only occur with this one attachment type. I should also note that the ex2k3 front end server is still running Windows 2000 with IIS 5.0 because it was a direct upgrade from Exchange 2000. (we are in an interim upgrade mode from Exchange 2000 to Exchange 2010, we did not want to upgrade the OS on this old/fragile server when it will only be in use for a few weeks). Anyone have any ideas?