Outlook Zipping Attachments

I have a client with Outlook 2002 for his email client. When he sends attachements, they are automatically zipped and he does not want this. I see there are many utilities to do this, but he says he has not installed any and I cannot see anything in the program list or Add/Remove that stands out. I also looked in the Registry for Outlook Add-ins, but nothing really stands out. Outlook Express does not do this. Can someone point me to a file or Registry area to try and stop this madness?