Here is the process when sending emails through Kennel Connection:
When you hit Send or it's time to send automatic emails, KC logs into your provider's server. Once the login information has been authenticated, it sends all emails to your provider. That is all it does. It's up to your provider to send the emails to your clients.
This error means that your provider is unsure if the app you're trying to connect from (Kennel Connection) is legitimate or not so they want you to log into your account to verify. If logging into your inbox via your web browser does not allow KC to connect, you'll need to look at your provider's settings for less secure apps and enable it. You also need to go in KC and click on File, Business Info, Email tab and make sure you have the username as your full email address.