The email I got was as follows and seems to indicate epub is also a problem now:
Dear Kindle Customer,
The following document, sent by you at 04:11 AM on Mon, Sep 25, 2023 GMT , could not be delivered to the address you specified:
* Runaway Jury - Grisham_ John.epub
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
JPEG (.jpg), GIF (.gif), Bitmap (.bmp), and PNG (.png) images
Additionally, Send to Kindle emails now supports EPUB (.epub). Later this year, we’ll also be adding EPUB support to the free Kindle app for iOS and Android devices and the Send to Kindle desktop app for PC and Mac.
If the format of the document that failed is supported (listed above), please ensure the document is not password protected or encrypted, and try sending it again.
You can learn more about sending personal documents to your Kindle in our product info pages:
https://www.amazon.com/sendtokindle
If you would like immediate assistance, please contact customer support at +1-866-321-8851 (US customers) or +1-206-266-0927 (International customers).
Sincerely,
Amazon Kindle Support