The RadEditor control generates proper XHTML code. Which can be rendered successfully in mail clients. Although you should be aware that stylization that comes from CSS files or references should be specially handled, so that the styles could be sent along with the HTML content.
Mostly when it comes to email CSS inliner tool is used. This tool is a simple external third-party library that transforms the CSS in the style tags to inline style attributes with the appropriate values. I suggest incorporating such tool with the RadEditor, so that you should be sure that the the custom stylization of the HTML content is correctly rendered by the email client.
For your convenience I am attaching a sample project, in which is integrated such tool, so that you could examine it further.