Well, Outlook Express certainly qualifies as non-Outlook. Outlook Express
definitely will ignore Rich Text attachments. That's documented. Moreover,
even if you're composing in HTML, it could still be that the recipient is
getting Rich Text. Is this person in your Contacts folder? If so, open the
contact record and double-click the email address. Examine the "Internet
format" drop-down at the bottom and make sure it's not set to Rich Text.