Need Outlook 98/2000 Exchange 5.5 guru

I have a user who is sending attachments to a rep and selecting this rep from his contacts folder. When he sends the attachment, the attachment is being stripped from the email. When he physically types in the reps email address in te TO: field, theattachment is received flawlessly. To make matters more confusing, if someone else tries from their machine to send the same attachment to this same rep and pulling the email address out of the same contacts folder, the attachment is being received with no problem.

Can anyone explain why this might be happening? I believe both of these users are using Outlook 98. The only real difference is that one system is a laptop and the other is a PC.

HELP!!!