Returned mail: User unknown

The original message was received at 2007-09-13 20:41:16
-0700
from postoffice.local [10.0.0.1]

   ----- The following addresses had permanent fatal errors
-----
<jabackesmac.com>

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