Send an email with a HTML file as body (C#)

Just set the MailMessage.BodyFormat property to MailFormat.Html, and then dump the contents of your html file to the MailMessage.Body property:

using (StreamReader reader = File.OpenText(htmlFilePath)) // Path to your 
{                                                         // HTML file
    MailMessage myMail = new MailMessage();
    myMail.From = "from@microsoft.com";
    myMail.To = "to@microsoft.com";
    myMail.Subject = "HTML Message";
    myMail.BodyFormat = MailFormat.Html;

    myMail.Body = reader.ReadToEnd();  // Load the content from your file...
    //...
}