Thanks for joining this thread, Sonny.

The investigation about the error when sending an invoice by email is still in progress. Since you've already tried all the troubleshooting steps recommended by my colleague JonpriL, I suggest contacting our QuickBooks Care Team if you haven't. They can add your company details to the list of affected users and get notified via email for the updates of the investigation.

Instructions on how to reach us are provided above. You can also have this investigation number INV-38793 as your reference. We'll be also updating this thread once we receive progress of the current investigation.

Feel free to reach back to me if there's anything else you need to know about QuickBooks. Have a wonderful day ahead!