This worked for me. Here is the process.

In NEW OUTLOOK there is, what looks like, an on/off button in the top right corner. Clicking the button will turn New Outlook off and return you to the original Outlook.

Microsoft asks for a reason you are switching back. If you reply, "New Outbox does not work with Intuit Proseries batch email." Maybe they will coordinate it better next year.

My checklists showed up in the old outbox as usual. (I deleted them because I had generated some duplicates. I sent the whole list again and it worked as expected.)