Hello there, Rudston.
Let me guide you in get getting the best help in fixing the error wherein you can't send emails from your Intuit QuickBooks Developer Sandbox. I recommend contacting our Intuit Developer support to investigate this further, as this case involves company email, which needs a private support session.
You can visit this link to get support directly from Intuit Developer's Support: Intuit Developer Contact Support.
Furthermore, you can also consider visiting the Intuit Developer QuickBooks Sandbox Community page. You can ask your questions in there, which is open to the public, and any users can reply.
Please feel free to leave a reply if you have additional concerns besides QuickBooks Sandbox. We're always here to help.
