I am using Ruby on Rails to generate the email. As mentioned in the Email markup docs, i modified my (*.html.haml) templates to include the schema for Email markup.
I had this same problem. It turns out I had a few misunderstandings which I will help clarify:
1) As pointed out above, the "Original Message" you saw in gmail was encoded as quoted-printable. You first have to decode the message, e.g. using Quoted Printable Converter, and the decoded message does not contain any 3Ds. The decoded message will then validate correctly (presumeably) using the Email Markup tester you referenced.
2) I arrived at this thread because I was debugging my integration using email markup. The markup wasn't showing in test emails and I thought I was implementing it incorrectly. I discovered my email schema integration must be whitelisted by Google to use these features.
I hope this helps.
you can try adding this meta tag in header
<meta charset="charset=ISO-8859-1"/>