URGENT unable to relay mail

Exchange 5.5 server all of a sudden it is not receiving mail.

The original message was received at Wed, 30 Jun 2004 11:32:28 -0400 (EDT)
from IDENT:mirapoint@[216.168.230.180]

—– The following addresses had permanent fatal errors —–

(reason: 550 5.7.1 Unable to relay for lheisey@microplasticsinc.com)

—– Transcript of session follows —–
… while talking to mail.microplasticsinc.com.:
>>> DATA
<<< 550 5.7.1 Unable to relay for lheisey@microplasticsinc.com 550 5.1.1 … User unknown
<<< 554 5.5.2 No valid recipients --------------------------------------------------------------------------------