I was just tweaking some code that sends out email. I noticed in Gmail that the plain text version of the email was being shown even though it was a multipart message with both plain text and HTML alternatives provided.

 

MailMessage msg = new MailMessage();
…
System.Net.Mail.AlternateView htmlView;
System.Net.Mail.AlternateView plainView;

htmlView = AlternateView.CreateAlternateViewFromString(
           htmlBody, 
           new ContentType("text/html"));

plainView = AlternateView.CreateAlternateViewFromString(
            plainBody, 
            new ContentType("text/plain"));

msg.AlternateViews.Add(htmlView);
msg.AlternateViews.Add(plainView);

The fix was simple: add the plain text view to the message first.  Yup, that’s all it took:

msg.AlternateViews.Add(plainView); 
msg.AlternateViews.Add(htmlView);