Unable to open attachment

I am running Outlook 2000 on Windows 2000 SP4, with an Exchange Server 2003 Enterprise Edition. The problem is, when I attach something and then email it to someone- they cannot open the attachment!? Do you guys think there is a restriction on my user account/mailbox ( how do I fix it) or is it something in my Outlook ( how do I fix it)- HELP PLeasE!!!!!

Another thing- very weird,
When I send an email with any attachment to this particular person, he cannot open it. If I send to someone in the office or outside of the office, they can open it. Help….