Calibre: can't email to my Paperwhite

I have Calibre set up to email a book from my library to my Kindle using my Kindle email address. The test email works and I get a 'verify' email from Amazon but shortly after, I get this email error message:

Dear Kindle Customer,

The following document, sent by you at 12:48 PM on Thu, Oct 06, 2022 PDT, could not be delivered to the address you specified:

Send to Kindle supports the following document formats:
Adobe PDF (.pdf)
Microsoft Word (.doc, .docx)
Rich Text Format (.rtf)
HTML (.htm, .html)
Text (.txt) documents
Archived documents (zip , x-zip) and compressed archived documents
MOBI (.azw, .mobi) (will not support the most up-to-date Kindle features for documents)
JPEG (.jpg), GIF (.gif), Bitmap (.bmp), and PNG (.png) images.

I used Calibre to convert to MOBI first as ePUB fails. What might I be doing wrong? I also tried converting to AZW,

Thanks for any assistance