- Create an email account on www.gmx.com
- Open "Settings" and check "Enable access to this account via POP3 and IMAP"
- Open "Preferences"
- Open "Sharing books by email"
- Click "Use GMX"
- Add GMX credentials
- Open Kindle "All Settings"
- Click "My Account"
- Copy "Sent-to-Kindle Email"
- Open Calibre.app "Preferences"
- Open "Sharing books by email"
- Click "Add email"
- Add Kindle Email
- Go to www.amazon.com/myk
- Click "Preferences" tab
- Click "Personal Document Settings"
- Add GMX email to "Approved Personal Document E-mail List"
- Open Calibre.app
- Right click on a book
- Select "Connect/share"
- Select "Email to..."
- Select your Kindle email