as far as i know a E-Mail can be formatted as Html or Text and a richtextbox is formatted as rtf (because rtf means RichTextFormat).
At first you have to make sure, that you E-Mail is formatted as Html.
You can access the rtf string in the RichTextBox.Rtf Property, there you can see how a rtf string looks like.
There is surely a possibility to transform a rtf to html, you can look for a possibility on the internet, but i hope i could help find a way to get a answer.
Sorry for my bad english
Edit:
There is a article for RTF convertion :)
Look here:
Writing Your Own RTF Converter