When you send emails through Kennel Connection this is the process.
When you hit Send or it's time to send automatic emails, KC logs into your email provider's server. If your login information is authenticated, KC sends all emails to your provider at once. This is all it does. It's up to your provider to send the emails to your clients.
This error means that your email provider is not allowing KC to authenticate to their server. You will need to contact your provider for assistance.