Exchange 2003 out going mail issue

Hello,
I have an Exchange 2003 enterprise server with about 40 users on it. 10 of the users are remote users. FOr remote users Ihave setup and configured RPC over HTTPS. One of the remote users can not send to anyone outside of the company. THey receive and NDR of:

The following recipient(s) could not be reached:

XXXXXX@XXXX.com on 2/12/2007 10:58 PM
550 5.7.1 Unable to relay for XXXXX@XXXXXXX.com

Are there any setting I should double check. Everything was working fine then this little bug came up. Any suggestions would be great.

Thanks