This means that we were unable to authenticate the connection to your SMTP server. Most likely, your password had changes recently and you just need to authenticate your new credentials.
To do this, go to your SMTP Settings where you should see the same error message:
Update your credentials and hit Authenticate and Save to see a confirmation message:
Then, go into your Failed Deliveries to try resending those emails. If you continue run into trouble, please submit a support request here.