Incoming email attachments are zipped

We use MS Outlook 2003. A user has attached a .PPTX file to an email and sent it to 30 users. Out of the 30 users, 5 of them receive it as a .ZIP file and when you open it, it contains .XML, .RELS, .PNG and .JPEG files (533 files to be precise). Is there a setting somewhere to stop this from happening? We have the Compatibility Pack for the 2007 Office System installed.