Outlook Attachments become win.mail

Often, my Outlook attachments are converted to win.mail files at the other end which no one can open....yet I know I was sending WORD or Excel Docs with a .doc suffix. I am working with a Dell Inspiron 1100 and Outlook 2000 SR-1.

This is intermittent, which makes this very tricky to solve. It is not the receivers either...