MS Outlook will sometimes send attachments wrapped up in winmail.dat files. This is not a common format, and other users may not be able to open these files.
If you are using Outlook - avoid sending emails in the RTF (or Rich Text) format. Instead use the universal standards of HTML.
To change this setting, in Outlook 2003...
Step 1: Under Tools/Options...
Step 3: Mail Format Tab, Select HTML
Step 3: Click the "Internet Format..." button and select "Convert to HTML format"
