We have to level with you: If you're sending e-mail to people outside your company, or, if not everyone inside your company uses Outlook, Rich Text Format (RTF) is not the best choice for a message format. RTF increases your chances of sending gobbledygook because it's the least compatible of the three available message formats.
Not that RTF can't be good. One benefit of this message format is that you can show attachments inline. This can be useful, for example, if you want to send a collection of documents in a message and include notes about what each document contains.
But, even this benefit is only useful when you're sending e-mail messages to people who are using Outlook running under Microsoft Exchange Server. Otherwise, you're better off forgoing RTF and sticking with HTML or Plain Text.
If you're curious about blocked attachments, see “Blocked attachments: it’s about security” later in this course or the "Useful links" section of the Quick Reference Card.