I have an open investigation with QB: Here the latest update  got from them:

Current status:

The engineering team have been able to confirm that through the status of the transactions continues to show as undelivered, our system reattempts to send out the emails, which are then successfully delivered.

They have advised that as a result of their analysis they have designed an in-product fix so that the status of the transactions changes upon successful delivery through re-attempt.

The team estimate confirmed that the fix should be ready to implement in product during our next QuickBooks release, which is estimated for the first half of June. 

If the estimated time frame changes at any point we will update you immediately. 

Workaround:

In the meantime, while the fix is being worked on please continue to use all available features, including manually downloading the invoices and sending them from outside of QuickBooks using read receipt, as a workaround.