@davidfor & @kovidgoyal
Thank you so much for trying to figure this out with me.
Obviously, I can re-create this consistently, so... to do a clean test I created a new Calibre Library. Then restarted Calibre.
Checked the settings (images attached).
Then I went to Amazon, "Manage your content and devices", and chose "download & transfer via USB" on a book I have purchased.
The book was downloaded to my Downloads folder, so it was auto-added to Calibre & converted to MOBI instantly. (image is attached)
Then I chose Connect/Share, Email to..., and chose the Paperwhite alias. Received the "autoconvert to EPUB question" (image attached). Answered yes and email was sent. (job image is attached).
Next images show that the EPUB was created and that they do exist in my file system.
Also attached is an image of the email from Amazon saying it failed.
I'll save that EPUB and now manually convert the MOBI to EPUB and email. (I assume it will work.) And also save that EPUB. Not sure what I'd be looking for precisely though. Do you want me to post them here? Not sure if that is allowed.
I'm running:
Mac Mini M1 Monterey v12.3.1
Calibre 6.1
deDRM 10.0.3
(however this was all happening with calibre 5.14 and deDRM 7.1.0 - I upgraded yesterday in case that was the problem)
Here is an additional link to the images in case its easier to view them on Imgur:
https://imgur.com/a/W2bYUFY