Attachments not going to all recipients

One user can't get attachments from another user only the message.
Specifically, Excel Attachments. If forwarded through someone else, he
receives them. Even if the same one had been sent to him directly
previously and it didn't come with the message.

The sender is using outlook and the receiver is using Outlook express.
Everyone else receives the attachment. Our belief is that the trouble is on
the sending machine. We are using a Coblt Cube (Linux) mail server.

No one else is reporting this problem.

Any ideas??