I use the Deferred Send Message Option very frequently.  For some unknown reason this week my Outlook *.ost file has become corrupted.  This file was kept locally on my PC in a cache file.  An outside IT service we use, explained to me that I closed Outlook before it finished updating with our Exchange Server.  Long story short,  from our Exchange Server they placed a new cache Outlook file locally on my PC.  But, I find out, that all messages in the Outbox are stored locally and not on the Exchange Server.  Any ideas on how we might get the information, out of the old Outbox.  We had renamed the old file as outlook-old.ost file.

Look at ExMerge as a possible solution. This tool can import data from a pst file directly into a mailbox. Make sure you run ScanPST on you pst file before you use ExMerge.