Exchange 2000 – RFC Errors

I’m not really sure if this is an issue with our server or not, but here goes. We are running a Windows 2000 Server with Exchange 2000. Generally email is working fine and everyone can email to us and we can send to them. Except, two companies cannot email to us (we can send to them). The error they are receiving is as follows.

Delivery Failure Report
Your document : xxxxx
was not delivered to : x@x.com
because: Enhanced Mail System Status Code (RFC1893):.4.1

Have checked technet and it seems they have a number missing before the .4 part. Can anyone help??

Thanks

Andrew Ford
Network Consultant