Hi Francois7, thanks for your post. Just to let you know (and update this thread) - we don't currently have any reported issues with invoice delivery in QuickBooks. Do the undelivered emails share any commonality such as the same email domain (e.g. Outlook, Gmail, custom domain etc.) and do you receive all invoices if you CC yourself into these?
To troubleshoot, you can also refer to our guide on 3 solutions for when your customers aren't receiving your emails for further tips.