Outlook 2003 not honoring MIME types

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I'm generating multipart emails that have plain text and html bodyparts. One customer claims she is seeing the entire email content, not just one bodypart. Sounds like her Outlook 2003 is not interpreting MIME at all, but I see no setting to completely disable MIME in Outlook 2003.
She claims that she's viewed the email on other PCs in her office with the same result, so I'm wondering if their whole office has MIME turned off somehow.

Can Outlook 2003 be configured like that?