Take a look at HTML e-mail
] on wikipedia.
Converting RTF to HTML in VB.NET the Easy Way
] requires word to be installed on the computer, and if that's a requirement - I'd just drop the rtf editor and use word directly - possibly embeding it inside the application as a control.
Basically you message will have to be rendered as HTML, preferably referencing nothing that's not part of the message for rendering purposes.
You could do worse than using DevExpress Rich Text Editor
] as a replacement for the standard .Net rtf editor - it knows about html.
If you want to keep on using the standard .Net rtf editor - you'll be able to access the
Text Object Model
], and can roll your own html exporter using this unmanaged API.
If you don't have a lot of time on your hands, shopping around for an existing solution would probably be the best way to solve your problem.