I finally have a solution to QB 2022 desktop invoices not being sent via email through my client's yahoo webmail and my outlook mail accounts.

Yahoo webmail requires a special app specific password for third party apps to generate an email which will send through webmail. Once you get that specific password from your account on the Yahoo the website, save it on your desktop in a text or PDF file. Go to QB preferences and set up "send forms" preference for Yahoo webmail. (Note that this is not you main Yahoo password but a specific one generated through your Yahoo account) The first time you try to send an emailed invoice from the QB invoice window, you will be prompted for this special email password. The wording is confusing because it asks for you email password. There is nothing to indicate that this is not your main email password. You will get a message that your email has sent successfully. You will not have to re-enter this special password again until you have closed QB and reopened it. That is why you save a copy on your desktop so you have easy access to it the next time QB requests it. Otherwise you will have to go back to your Yahoo account and get another app specific password.

I have to do something similar when I use Outlook to email my invoices to customers. In this case my email is an iCloud email which requires app specific passwords to send and receive iCloud email through 3rd party apps. When I set up Outlook to handle my iCloud email I had to get a special password for Outlook by logging into appleid.apple.com and signing into my apple id account. The nice thing about Outlook is that I do not have to re-enter this app specific password each time I restart Outlook. Apple does require me to get a new one occasionally. The time frame may be annually but not often. To get QB 2022 Desktop to send emailed invoices through Outlook, I have to follow the Yahoo steps above and get a new app specific password for QB from appleid.apple.com. In QB preferences I set up "send forms" to use Outlook. Thus far, I have been able to email invoices just fine as long as I re-enter the special email password the first time I send an invoice via email.

If any of this is confusing let me know and after 4/15/22 I'll give more detailed steps. I have spent what seems like days trying to work this out with QB support but none of the support agents knew the whole answer. Some had figured out that the issue was tied to the extra authentication email services were implementing which gave me a clue how to solve the Yahoo and Outlook-iCloud scenario.