calibre-web

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