I rarely ever enter my customer's email into quickbooks. Under the customer's info I always enter my own email so that when I click "review and send" it sends their invoice to my email. From there I can copy the link to text it to the customer or forward it as an email to the customer (removing the fwd markers) That way it's coming from my own email address and not a third party. To me, it looks unprofessional coming from a third party. In my experience their janky system requires all these work-arounds to get it to function the way you need and then when they figure out you're going around their stupid system they make "improvements" to make it even less functional.

I think this is all about collecting as much data on your customers as possible. Used to you could just enter their zip code and it would pull all the appropriate sales tax but then they "improved" it so you had to enter in their full address (so they could collect more of your customer's data)