When attempting to use any kind of embed code in an HTML card in the editor, you'll see that this does not work as expected in a test email.  

The reason for this is because modern email clients automatically disable embeds for security, including iFrames and any type of embed that uses JavaScript.

Since this is a limitation that is entirely controlled by external email clients, there is no way to make a dynamic embed work in an email. However, there is a workaround which some people use to share dynamic content in email newsletters:

  1. Take a screenshot of the embed, once added to the post
  2. Insert the screenshot as an image card in the post (in place of the embed)
  3. Use the link feature tied to the image card, to turn the image into a link
  4. Publish and send the email newsletter to your members
  5. Once the post is published and sent, you can optionally swap out the screenshot for the embed, which will be available for any visitors to that post on your site

By default, Ghost automatically renders embeds for YouTube and Vimeo in your email newsletters so that readers are able to click on them and open the video as a link.