Outlook user receives messages differently from two discussion lists

I administer a domain that has two discussion lists, and one of the members who is subscribed to both of them says for one list, his Outlook client shows the content normally, but for the other list, it shows as a bunch of attachments.

I have carefully compared the settings for both lists, and they are identical except, of course, for things like the name of the group, etc.

Does anyone know why two lists with identical settings would show differently when received in Outlook?