Just corrected a stupid issue on an Exchange 2007 server where my users were sending emails to external recipients in rich text format (RTF) - even though the users were specifically choosingto send emails in HTML format. And part of what makes RTF a terrible format for email is that non-Outlook users receive all emails as winmail.dat files - which is how this problem presented itself. There is a setting in Exchange 2007/2010 where messages sent to certain external recipients in certain situations will always be converted to RTF no matter what the original format the message was sent in.
After investigating, I found this to be DEFAULT behavior in this scenario:
1) you are sending to an external contact defined in Exchange Management Console
2) the contact you are sending to does not use Outlook
To resolve it, you need to edit the properties of the contact. There is a field on the general tab for "Use MAPI rich text format." It should be changed from "use default settings" to "never"
You can also fix this universally for all users (by changing the default settings) in EMC > Organization configuration > hub transport. On the Remote Domains tab, right click on Default and choose Properties. Click on the message format tab and choose "Never use"