Server Response 553

Wanted to know what this message actually means i got it from my college friend LAN. I believe hes using Exchange 2000. Only change was done a Day or two he wanted to restrict email from being send and recieved from a specific email address. Does this have anything to do with Blocking Specific emails recipients?

The message could not be sent because one of the recipients was rejected by the server. Server response: 553 sorry, relaying denied from your location [xxx.xxx.xx.xxx], 9#5.710?.(account: ?mail-a-xxxx.com, SMTP server mail.xxx.com?error number: ox800ccc79)