这是微软MSDN公布的Fix，希望对碰到同样问题的朋友有所帮助：
This appears to be inaccurate. The AlternateView class does not have a
constructor that takes a string as the first argument and a media type
as the second. This throws an error when I try it, because it treats
the body variable as a file name and attempts to find said file.
Here's the fix:
AlternateView alternate = AlternateView.CreateAlternateViewFromString(body, null, MediaTypeNames.Text.Html);
Courtesy of Mike Pope's site:
http://www.mikepope.com/blog/AddComment.aspx?blogid=1264