Like many other service providers, SocketLabs On-Demand services require a username and password to access your account. The above response from our SMTP servers indicates that the credentials you have sent to our servers do not match the settings we have associated with your account, and thus you will be unable to send messages via SMTP until you have provided the correct credentials. Please be aware that after a certain amount of incorrect authentication attempts, our system will block the IP address being used for a few minutes for your protection.
Your SMTP Credentials can be found online in the SocketLabs On-Demand Control Panel. Select your server, then go to Configuration > SMTP Credentials > View SMTP Credentials.
If you are using our SocketLabs On-Demand Service as a SmartHost for relaying mail from your own server, please see the following article for more information on how to properly handle some common scenarios: How to increase SMTP performance when sending messages through Socketlabs