Distribution List Attachments

When I attach an Excel file to a 240 member Outlook 2002 (XP) distribution list, the attachement is either recieved as sent, as a .dat file, an excel file that will not open, or not at all. I have also run into the same problem with Word files. The file is usually between 80KB and 1.5MB. My customers are really frustrated, as am I. Thanks for any help.