Email form a client is blocked in Exchange!!

Hello Everyone,

A short while ago, we started hosting our own Email server(Exchange 2003). I had called our ISP and got them to point their MX records to our new server.

Everything seem to be fine except that one external client(US goverment related) cannot send email to us. Any email sent to us just doen’t come through.

Their IT people think it’s on our end.
I didnt find anything in the Queues in Exchange System manager. Is the problem on our Server or our ISP provider.??

Any idea ??
Many thanks!!!

E. Canaan