Returned mail: User unknown

The original message was received at 2009-08-12 16:04:58 +1000
from postoffice.(null) [10.0.0.1]

----- The following addresses had permanent fatal errors -----
<snapper88@optusnet.com.au>

-----Transcript of session follows -----
... while talking to postoffice.(null).:
>>> RCPT To:<snapper88@optusnet.com.au>
<<< 550 5.1.1 unknown or illegal alias: snapper88@optusnet.com.au
550 <snapper88@optusnet.com.au>... User unknown