What happens if you save the attachment to a local store BEFORE attempting to edit. Files opened from Outlook store in a TEMP directory and that directory should not be used for long term storage.
Are you attempting to save the modified file locally or to a network drive.

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

Great. Have the latest SP applied. The unscientific method of checking network performance is to work on the file locally (your laptop) and then move them to a network drive after the work is done. See if that solves the issue.