The Video is coded with HTML5 and will work in Apple mail, iPad and iPhone. Other mail clients see a fallback image of your choice.
MailChimp don’t accept HTML5 video’s, it’s impossible to send an email with video through their system. If you export your file as a MC file you will see a normal 310 pixel fixed width image (like the fall-back image). The HTML5 video is working when sending your email through Campaign Monitor, Constant Contact, or some other Email platforms.
You can check the Autoplay button if you want to send your Email with a Autoplay function.