Kindle EPUB issue

Forgive me if my question seems dumb - I'm new here.

For some time, I've been using Calibre to e-mail documents to my Kindle Paperwhite reader in the MOBI format. Recently, a transfer triggered a message from Amazon announcing that effective 8/22, they would no longer be processing transfers in the MOBI format, and suggesting that future transfers be done in the EPUB format. So I had Calibre convert the documents I had in backlog to EPUB, but when I try to e-mail them, I get an error message from Amazon stating that the EPUB format isn't recognized.

I tried contacting Amazon, but the chat person in India wasn't helpful.

Anyone know what is going on here?

Thanks