Winmail.dat

When a user in one of our offices is sending an attachment through Outlook 2000, to other users using Outlook 2000, all attachments he sends are converted into Winmail.dat files. What causes this and how can we stop this happening, It also happens locally in one office on occasions.

Our mail software lets ZIP files through without any problem, and other users using 2K are able to send me files correctly, we have checked MIME settings and changed from FTF to Plain text. Still the problem exists.