Readbetter needs permission to send emails to your Kindle. This is done through Amazon's Personal Document Service. Readbetter will never send emails to your Kindle without your permission.
Go to Manage Your Content and Devices on Amazon.com
(If you are in another country and Amazon.com is not your main Amazon account, navigate to your Amazon settings, then find the menu item "Content and Devices")
Navigate to Preferences and then scroll all the way down to Personal Document Settings.
Under Approved Personal Document E-mail List, click Add a new approved e-mail address
Enter @readbetter.io
and click Add Address.
On the Settings Page, just above the Approved Personal Document E-mail List, you'll see your Send-to-Kindle E-mail Address. Copy the email address for your Kindle. (usually [email protected]) and paste it into the form at the bottom of https://readbetter.io/app/setup
Hit Save.