You could get content from Word or Excel (or any other app) in html format end this would be ideal for email :
string data = Clipboard.GetText(TextDataFormat.Html);
You will get html string but with some header in front, look at the links below for examples how to handle it :
http://blogs.msdn.com/b/jmstall/archive/2007/01/21/html-clipboard.aspx
http://theartofdev.wordpress.com/2012/11/11/setting-html-and-plain-text-formatting-to-clipboard/
warning html produced by word is simply horrible so probably there will be problems for some people when viewing such mails with messy html.