MS word attachements not working.

we have a group of user’s in one location that are unable to to send word doc’s from outlook 2000. The end-user are able to insert any file except MS word docs in an e-mail mssg and the system will send it but as soon as you insert a word doc then it will not send. And its only affecting one zone in our building the funny thing is when they go to another part of the building it will work. We are running exchange 5.5 on NT4.0 with all clients using Office 2000.