The solution you provided does not work and sending a pdf is an inelegant approach requiring unnecessary work.
After nobody responded for many months I finally managed to identify the problem and a solution.
Gmail does not accept the Quickbooks transmission saying it comes from an insecure browser. I presume this is because Quickbooks is using its proprietary browser.
Gmail offers a way to generate a more secure App Password which can then be used in Quickbooks each time an invoice needs to be sent electronically.
This is, while inelegant, superior to generating and separately sending a pdf or waiting for Quickbooks to acknowledge the problem.
