Hi Julie
When I attempt to use the save as feature on the .mpp file opened directly from the Outlook email attachment, I do use a local folder, not the temporary directory. So I don't understand why I am getting the error.

If I attempt to save the .mpp email attachment to a local folder before editing, and then open the locally saved file, it works fine. I can edit the locally saved file and re-save. However, that is no use if you have just spent an hour updating a file opened directly from Outlook and then cannot save the changes.

So yes, there is a workaround. But that doesn't actually resolve the problem.

Thanks