d740b30b_Known_Issues___Computer_Help__Issues
[Workaround] Sometimes Outlook will send a message using the RTF format (which is not very common outside Outlook) for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file. For Outlook 2007: Select Tools | Options... from the menu. Go to the Mail Format tab. Under Compose in this message format:, make sure either HTML or Plain Text is selected. Click Internet Format. Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format: Click OK. Click OK again. For Outlook 2010: Select the File Tab | Options... Select the Mail button and under Compose Messages Select HTML under the Compose Messages in this format Then scroll down to Message Format Make sure Convert to HTML format is selected under the When sending messages in Rich Text format to Internet recipients option. Click OK [Issues] 
In Outlook 2010 how to prevent winmail.dat attchements - Does winmail.dat show up for some but not all recipients of your messages?
Known Issues | Computer Help
Issues