Video isn't widely supported in email clients. Fortunately, there are a couple of other ways you can add a video into an email.
Insert a video thumbnail image
An image of the video (this could be a screenshot of the video with the play button) can be inserted into the email. If you host your video on YouTube, the video can be set to autoplay so as soon as the recipient clicks on the image they’re taken to the video and it plays immediately.
Insert the video image by adding an image into the email template, drag and drop the image into the target box, and click Save Element.
Add the URL of the video by clicking on the thumbnail image on the Content section of the email template builder, paste the URL into the Hyperlink box.
2. Use an animated GIF
You can also insert an animated GIF instead; this works in the same way as inserting an image within an email template.