Winmail.dat problem in Outlook 2002.

I use Windows XP/Office XP pro, and am having a problem with Outlook 2002 (XP).  Whenever I email an attachment to a non-Outlook user, they receive my attachment as "winmail.dat".  I am pretty sure this means that Outlook is sending the email using rich-text format (RTF/TNEF).  But I am not using rich-text format!  My mail format is set to html -- I've tried plain text, too.  And under the setting called 'When sending Outlook Rich Text format.. use this format', I've tried both 'Convert to HTML' and 'Covert to Plain Text'.  In all cases, my messages headers do inlclude a X-MS-TNEF-Correlator.  

 

I've also tried changing the properties for my recipients under 'Contacts', by setting the Internet format to 'Send plain text only' and 'Let Outlook choose...', but these don't seem to help either.

 

Is there some setting I am overlooking?  My goal is for non-Outlook readers to receive my attachments correctly.

 

Thanks!

 

Bruce.