I am too facing same issue. Need pointers urgently

I have 7.1.1_3196 RHEL 5 build.
I have enabled archiving to a third party domain. The relay to the specified domain was working fine but after I restarted the box, getting -
"postfix/error relay=none dsn=5.0.0 status=bounced"
error and relay to third party domain is not working.
Delivery to local domain or other domain is working fine.

Please provide pointers to resolve the issue.