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:
Content-Type: multipart/alternative
or
multipart/mixed
. For each part, the content type should be a valid MIME type, such as
text/html
.
Please see:
http://en.wikipedia.org/wiki/MIME[
^],
http://en.wikipedia.org/wiki/HTML_email[
^].
Sending e-mail in .NET:
http://msdn.microsoft.com/en-us/library/system.net.mail.mailmessage.aspx[
^].
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.[
^].
—SA