[a_e_w]

I have the same problem. I have been using this feature for a long time without problems and have made no changes.
Now when I atempt to email from Calibre, the spinny wheel goes round for a few seconds and that's it. No document appears in my account (or my wife's account). No emails are received from Amazon.
(It doesn't work from my wife's PC either - using a completely differen't email account).
If I use the 'test email option', it reports success and that's followed by an email from Amazon reporting (correctly) there was no attachment.
If I email the book from the same account, via Outlook rather than Calibre, the book transfers successfully.
I uninstalled Calibre and installed a previous version, but the problem persisted.
That would seem to indicate that Amazon have changed something.