Thanks for letting us know all the steps you've taken to resolve the problem, @dmarbach.

Currently, we have an ongoing investigation (INV-58793) about the undeliverable or bounced email error when sending invoices. Intuit is working diligently to resolve this issue. They'll communicate more details once the research is complete.

I recommend contacting our Customer Care Support team so that you can be added to the list of affected users. This way, you'll receive email updates of the investigation's status and be notified once it's been resolved. I've included the steps to contact support below. 

1. Click the (?) Help icon in the upper right-hand corner of the Dashboard.
2. Select Talk to a Human.
3. Enter a short description of your concern and press Enter.
4. Click I still need a human.
5. Select Contact Us to connect with our live support.
6. Choose Get a callback.
   

You can visit our Community Help website. This website shares helpful articles to read on different topics about QuickBooks. Just select a subject from the Topics drop-down menu.

Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.