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 email provider's server. Once your login information has been authenticated, KC sends all emails to your provider at once. That's all it does. It's up to your provider to send the emails to your clients.
This error means that KC is unable to log into your provider's server.
You will need to go to File, Business Info, Email tab and verify with your provider that all of your settings are correct, including the outgoing server.
If they are, then you will need to work with your provider to figure out why their server is not allowing KC to authenticate to it.