[a_e_w]

Previously, you could setup Calibre to email books to Amazon by simply specifying your email address.
To be able to do that, I think the email must actually have been sent from Calibre's mail server with your email address set as the return address.
What I think has changed is that Amazon are now comparing your email address with the server address and rejecting emails where the two don't match.
The thing that's causing confusion is that they seem to be carrying out this check after they've carried out the check to see if there's a valid attachment.
When there's no attachment, you get an email back that they've rejected it, when the host names don't match, you get nothing!

You don't need to create a new mail account, just enter the settings for your current mail provider's SMTP server. To find the values to enter into Calibre, just do a search for the relevant settings, e.g. 'gmail mail server settings'