Can’t send attachments w/ outlook 2000

We are using NT 4.0 pak 4. With exchange 5.5. We use Elron Firewall. The mail server is in the firewall. We have a WAN with three domains (webserver, olc-net, and olc_admin) The exchange server is on the webserver. The exchange server is located in the LRC. We are using Outlook 2000 and Outlook Express. The people having the problem or in another building with a 100mbit fiber transciver connection. People in the LRC are able to send attachments fine. Is this a problem with outlook? Would Netscape mail work ?