A quick google search would reveal the reasoning for your error message.

http://www.aota.net/Troubleshooting/error553.php4
This error is caused for one of two reasons:
1) You are attempting to send email to a domain that is not recognized by this server
2) You are attempting to relay email through this server, and have not authenticated
The reason in bold is the most likely reason for this error. If you read further down on the page you will see the SMTP AUTH command. I would suggest making use of this before attempting to send email, or else check your POP3 mail before accessing the server.

Why do you need to access the server directly? Use your e-mail client.

BTW if this is a mail server you don't have permission to use, then you will never be able to authenticate and never be able to send the mail.