Do you see your emails in Failed Delivery with an "Authentication Unsuccessful" error? This means that we were unable to authenticate the connection to your SMTP server. Most likely, your password changed recently and you need to authenticate your new credentials.
1. Go to your SMTP Settings where you should see the same "Authentication Unsuccessful" error message.
2. Update your credentials and click Authenticate and Save.
3. Go to your Failed Deliveries and try resending those emails.
If you continue run into trouble, please submit a case for Support.