receiving .doc files as .dat

A user sends out email (using Outlook 2000) with an MS Word2000 .doc file attachment. Most recipients receive this attachment as it should be, that is as .doc. The problem I’m facing is that some of his recipients receive the file as .dat file… The only possible common thread is that some of the recipients use Eudora mail. But knowing that hasn’t given us a clue on how to fix this problem.