Outlook 2007 winmail.dat

When you sending message every recipient have setting 'Internet format'
This property determines how Outlook will convert message to MIME format while sending it to internet recipient.
So
While composing message right click on the recipient and select 'Outlook properties'
In the shown dialog select 'Send plain text only' for 'Internet format'
press OK

If recipient is added from your Contact via Outlook Address Book you will see the contact. So right click on the contacts email address and also select 'Outlook properties'. Set format to the plain text only.

With this settings Outlook will send multipart email message.This meesage will include plain text message and HTML one.

Hi,
Thanks for your responses, the individual contacts are definitely set as HTML, not RTF.

If I set it to  "plain text only", it does not send the winmail.dat file, but I need to send in HTML.

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

Sending as HTML should not cause this problem.

Are you using Exchange? If so, how's the Message Format configured for internet mail?
(tab Message Format - Message Encoding, e.g. MIME, provide message body as ..)?

Nope, not using exchange

Is there any custom addins for Outlook?
For just Outlook it does not matter how message is composed.
When sending to internet recipient Outlook checks recipient options.
So if recipient is marked as it can't receive Rtf message won't be sent in RTF format.

All information is copied from Contact's data when you adding recipient.
But this local copy can be modified by custom software while sending message.

Hi,
Nope, no addins either, contact info is definitely set to HTML

Can you compose test message save it as Outlook.msg. and post it zipped here. I can look at it's properties.

Any clues in the internet header (received mail)  on any potential culprit?

Hi,
The attached file is actually a .msg file, just rename it.

I don't see anything in the header that points to a potential issue.

Thanks for your help

Blank message with test in the body but no addresses or header ..  that's not what the received received; need actual message (you may anonymize sender/receiver/subject info)

sorry about that,
I actually did it properly this time and noticed that the content type is multipart, on the receiving end. But I am sending it as html, so it should not be multipart, any ideas?