Sudden unexplainable Zimbra / Shaw Webmail "Could not send message: mail.SEND_FAILURE" error

Problem:  As title.  Probably affects only one email draft, others are fine.

Possible solution:  You might have attached a Windows shortcut file (i.e. *.pdf.lnk) rather than the actual intended file (*.pdf).  

It seems Zimbra tries to follow or validate that shortcut before sending, and cannot, and so barfs the "SEND_FAILURE" error.