To remove this error, you need to login to your pop account and check your mail first. The server has anti-spam software installed that won't allow anyone to use the SMTP server until they have logged in first. This keeps just anyone from connecting to the server and sending out thousands of messages. You may need to wait 20-30 seconds before trying to send mail again for the server process to recognize you after you check your mail.
Once you have logged into the POP server and have been authenticated, you will be allowed to send mail for 30 minutes without having to login again. If you do login again, your 30 minutes will start from the time you logged in last.