Download Error when Opening Attachments
If you're receiving a download error when you try to open an attachment from Postbox, it could be trying to use an old version of the associated application.
The error could state:
"[the attachment] could not be opened, because an unknown error occurred. Try saving to disk first and then opening the file."
Here's how to resolve this issue:
On Mac: Go to Postbox | Preferences | Attachments
On Window: Go to Tools | Options | Attachments
Then, find the content type that's causing the error (.doc, .pdf, etc.), and choose the correct application in the Action column. Even if it looks to be correct, the application needs to be re-selected.