Basically, you need multi-part e-mail; and one or more parts could be HTML. Or RTF, which I would not recommend. For the e-mail, the content type should be:
. For each part, the content type should be a valid MIME type, such as
Sending e-mail in .NET:
For more complex cases where one part can reference another part, such as image referenced in HTML, please see my recent answer:
Sending HTML attachment with images
The source code in this article also shows how to compose multipart e-mail using C#.
Please also see this CodeProject article:
Sending an Email in C# with or without attachments: generic routine.