Exchange – Recipient name not recognized

I am recieving the following error when trying to send email to one particular address. All other email goes out and comes in just fine. Has anyone seen this before or have any ideas? Running Exchange 5.5 sp4 on NT 4.0 SP5.
Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 10/16/2001 1:39 PM

The following recipient(s) could not be reached:

‘rfitton@rgm.ca’ on 10/16/2001 1:39 PM
The recipient name is not recognized
The MTS-ID of the original message is: c=US;a= ;p=Group Publishing;l=LONGS-011016193908Z-63926
MSEXCH:IMS:Group Publishing:Loveland:LONGS 3500 (000B099C) 500 5.7.1 – You have a server error