Returned mail: User unknown

The original message was received at 2004-08-27 11:24:09 -0600
from postoffice.local [10.0.0.1]

   ----- The following addresses had permanent fatal errors -----
<>

   -----Transcript of session follows -----
... while talking to postoffice.local.:
>>> RCPT To:<>
<<< 550 5.1.1 unknown or illegal alias: 
550 <>... User unknown