Smtp: Invalid Response Code Received From Server

i have ispconfig version 2 installed on my linux centos 5 server. everything was working find then all of the sudden when i was testing out my script, i got this error:
Failed to add recipient: example@example.com [SMTP: Invalid response code received from server (code: 550, response: 5.1.1 : Recipient address rejected: User unknown in local recipient table)]

what was able to find out: well, as i troubleshooted, i learned that the recipient email address was also a domain on the same server, so i think this is why it was giving me this error, because once i changed the email to a different email address like yahoo.com - i didnt get the error anymore.