I think I may have this issue partially addressed, but going to spell it out just to make sure we're all on the same page.

Problem: When trying to email invoices, it asks for password. Entering my email password fails. By getting a "Secure Mail Key" from ATT/Yahoo/SBCGlobal or "App Specific Password" from other companies, I'm given a 16 alphanumeric code instead. When putting that in, it works to send emails again, but once QB is closed, and reopened, it asks for the password again.

This is not a solution, but rather a "We understand the problem and have a workaround"

Quickbooks, in order to use the "App Specific Password" or "Secure Mail Key" needs to have the "Use Enhanced Security" option turned on in the Preferences > Send Forms > Edit section of email. However, as of some time mid-2020, this feature is missing/broken. QB has indicated they're aware of the issue, and have BEEN aware for almost a year, and not fixed it. 

That said, there are two workarounds.

1) Keep your App Specific Password in a text file on your desktop, and copy and paste it every time it asks (once a day?). This is both obnoxious, and a security concern, but still a viable option.

2) Send invoices from a different email that doesn't use an App Specific Password. Best recommendation is to use a Gmail account, and turn on the feature called "Less Secure Apps". It functions in the same method but simply 'allows stuff' rather than create a new password. This is a similar process for using Gmail on Outlook for desktop.

I'm under the impression that QB simply isn't going to fix this until a proper amount of ire is raised, which may be never, since finding solutions and even identifying the problem is a project in itself.