The problem may be on the sender’s end of things. In the sender’s address book, they can indicate what type of mail system the recipient uses. Ask them to check if the settings for these two recipients are different. If they are, ask them to make them both the same as the one that works correctly.
The problem IS on the sender side. They use for one of the recipient the RTF format for attachment. The way to find where is different based on the version of Outlook. For isntance in outlook 2000 one must double-click the mail adress in the contact to get access to the format.
Google winmail.dat and you’ll find the (tons of) answer(s).