Most likely cause is a user property added to the message.
Add the following named property (known as UseTnef) and set it to false:

{00062008-0000-0000-C000-000000000046}, 0x8582, PT_BOOLEAN

Dmitry Streblechenko (MVP)
http://www.dimastr.com/
OutlookSpy - Outlook, CDO
and MAPI Developer Tool

"Mark Beiley" nowhere AT donotuse.com wrote in message
...
I'm using MAPI to send an email with my Outlook add-in. Sometimes the
emails are sent with a winmail.dat attachment when sending through an
Exchange server. I'm trying to figure out how to prevent this from within
my add-in? I've read this:

http://support.microsoft.com/kb/138053

but in my case I'm adding the recipients dynamically, and they aren't in
any address book. I also don't have control over how they've configured
their Internet Mail Connector (IMC).

I'm wondering if any of these will help me?:

PR_SEND_INTERNET_ENCODING
PR_SEND_RICH_INFO

Any tips would be greatly appreciated.

Thanks,
Mark
http://www.email-announcer.com