Emailed Invoices through Timeslips not making it to my Sent box in Outlook

I use the automated emailing for invoicing through Timeslips 2016.  Even though my log report shows "Success" for the emails, I only get a few put back into my "Sent" box in Outlook, so I am not confident that they were delivered successfully unless I get a response or a read receipt.  Is there a way to retrieve these?  (We save all of our invoice email correspondence in public Outlook folders for future reference.)

Thanks,

Debi