Additional Configuration for Kindle E Mail - janeczku/calibre-web GitHub Wiki

If you wish to send books to your kindle, not only do you have to add your "send to kindle" email address in Calibre-Web and configure the e-mail server, but you also have to whitelist the email address that your server is sending emails under.

On Amazon,

• Go to Manage Your Content and Devices.
• From Preferences scroll down to Personal Document Settings.
• Under Approved Personal Document Email List, check if your email address is listed. If your email address is not listed, select Add a new approved e-mail address. (the one you have set in outgoing email name in Calibre-Web)
• Enter the desired email address and select Add Address. (Thanks to @funnelcloudservices for providing this little tutorial