Attachments on Outlook 2000

Does any one know what needs to be done for outlook 2000 outgoing attachments to be seen by the recipient?

I sent the attachment to a friend, but he could read the email but he could only see the attachment in a form some hex decimal codes all over the page. The recipients have office 2003 and I have office 2000. Is there a possibility his office 2003 may not recognize office 2000 documents?

Somebody please help.