In Exchange 2010, the following setting needs to be changed to prevent RTF emails from being sent to outside recipients:
1. Open Exchange Management Console
2. Open Organization Configuration
3. Open Hub Transport
4. Select the Remote Domains tab
5. Select the Default profile (Domain is *) and open its properties dialog
6. Select the Message Format tab
7. Select « Never use » under « Exchange rich-text format: »
8. Click OK or Apply