Same problem here since few days.
Calibre email test is ok, but I don't see mobi files on my Kindle when I send them via Calibre.
If I send mobi files as attache by my email account with "convert" object, they are immediately downloaded, so it's clear that Amazon doesn't accept anymore files sent by Calibre server.
I'll continue to use Calibre only like converter because I don't need to insert my email password in any other application
update: after sending a test mail by Calibre, a return mail from Amazon says that no attachments were in the message.....so, Amazon recognizes messages from Calibre server.
Hoping to be a temporary issue