Outlook recommends Mozilla

I received an email today advertising Mozilla in a somewhat subtle way: The X-message-flag of the mail was set to

You are using Outlook. May I recommend Mozilla (http://www.mozilla.org)?

In Outlook XP this results in Outook seemingly recommending me Mozilla:

!/journal/2004/11/18/outlook_recommends_mozilla/YouAreUsingOutlook.png (You are using Outlook)!

As far as I have been able to work out, Outlook it the only mail client that displays the X-Message-Flag upfront to its user. Interesting but somewhat intrusive and spam’ish way to advocate.