If you do ever end up in that situation again, (or someone else is,) you can download the notebook by moving it into a folder in OneDrive. Then go to the web and use the option to download the folder. That will zip up the folder, with the real one drive files inside.

You’ll still need to find an app to import them into your new note taker though.

He needs to use a mta that supports arc sealing and setup his Google workspace to accept the arc from his server. Pretty sure Google also supports connectors for email so he could even just have Google accept all emails from his ip. I suspect he is cheaping out and doesn’t want to pay Google for a business grade service.

I don’t care what he did in the past, it is obvious he is not an up-to-date email admin. Using free consumer accounts as your primary email, is not a defendable position for anyone other than the self employed.