.txt attachments with every mail message

Hi,
Something strange is happening now. One of the users is getting a .txt attachment with every normal email message (whatever message in the body of the email message is exactly the same in the .txt file). So he’s receiving an attachment with every single email, eventhough there’s not suppose to be attachment. He’s using Outlook 2000 from Office 2000 Professional. Can someone help us out please or let me know if you need more info.
Many thanks.