Not able to send to a spefic site

We are using Exchange 5.5 with SP4, the client is Outlook 2000 with SP1.
Any address at a spefic site we get the following error
Unable to deliver the message due to a communications failure

But we can send to it from an external account. And we can send to any other external system
I have check all my connector settings and do not see any place that states it is being blocked.
The address can be pinged and it can be resolved with nslookup.
Any suggestions would be greatly appreciated