I would just like to point out that the Quickbooks work around (I use that phrase lightly as its not really a work around) of downloading invoices and emailing them manually is fine for invoices you are manually generating.
We have automated reoccurring invoices for monthly rentals that are automatically sent out by QB these of course are not being delivered and we have no way of knowing that without going through all of the the reoccurring invoices every day and then downloading and sending manually.
I would also like to point out that QB does not show emails as undelivered, it shows them as delivered but they may not be received by the recipient.
QB are missing deadline after deadline for fixing this issue.
